I am posting this thread at the recommendation of Fear Itself. 
I need to know how to reinstall, or otherwise work on the drive for the mouse pointer. I have even gone to the Mouse Properties dialog box and clicked on the little box in Snap To, and still the pointer drifts to the lower left or upper right. It even does so in an uneven manner as if someone else were manipulating it!
Any help here? It’s a Dell Inspiron 4100 Laptop, a few years old.
Is yours (or, in addition to a built-in, are you using) a wireless mouse? My mouse pointer does that when the battery in the wireless mouse starts running out of juice.
No; it acted that way with the mouse pad on the laptop surface itself; and whn I installed an exterior mouse (optical, no less), it did the same thing.) 
Try disabling the mouse pad instead: the computer will take directions from both unless you do.
If you have the pointing stick in the middle of the keyboard, I would disable that unless you use it. On my Dell that gives the most problem with pointer drift.
It seems the amount of drift is cached, so when I let it drift for a long time it eventually stops fighting me. Not helpful if you don’t have the pointing stick, so sorry if you don’t.
Simply put, I don’t know how to disable the mouse pad. 
It may not even be an option to disable the mouse pad. On my paticular model it is FN+F1 but it may be different on yours. You might be able to disable it in the BIOS if you NEVER use it.
FN?
The Function key, usually labeled as FN or Fn in a different color than the normal keys, often blue.
This is the key to allow you to control various special functions, like turning the volume up and down, enable/disable the external display, etc.
So he means press the Function key and F1 at the same time.
I always just call it the “effin’” key.
This is really weird because I had this exact problem just a couple of weeks ago. It turns out that it was a bad touchpad. Disabling the pointing stick stopped the pointer from drifting, but the pointer movement was choppy.
To disable the pointing stick, go into the mouse properties dialog box and click on the device settings tab. There should be options to disable the pointing stick, touchpad and buttons. If those options aren’t there, you have the same problem I had. After I plugged in a regular mouse, those options were no longer there and I had to uninstall then reinstall the touchpad driver.
I ended up ordering a new palmrest assembly from Dell (go to support.dell.com then search “parts”). They’re $65.00. You can find the instructions to replace it online. You have to remove the keyboard and display first. The pointer drift went away and the touchpad works perfectly.
Thanks, eLunatic, I’ll chreck that out. 
And to the others of the Teeming Millions who mentioned or explained FN, or the Function Key, from the replies you’ve given I’ve looked the keys over and I find a row of what are called “hot buttons” at the top of my keyboard–above the F+number buttons. The first one is simply F in a box. The others show a computer icon, as well as icons for pages, W (probably for Word, not our President 
), X (Excel?), a grid, and the back of an envelope. Perhaps the f-in-a-box is the FN key you mean. Give the go-ahead and I’ll try it. * I must point out this is an exterior keyboard, not the one on the laptop itself!* I’ve just now noticed the Fn key on the laptop’s own keyboard, duhhhhh…:o
It turns out my Mouse Properties dialog box doesn’t have a Device Settings tab. The Buttons tab has a Settings button, but it always stays gray and is thus unusable.
I just checked on my computer and the Device Settings and More Features tabs have little Synaptics logos on them. You might try updating your touchpad driver, but before you plug in a regular mouse, go into mouse properties and try disabling the pointing stick.
On reading ChrisBooth12’s suggestion, I don’t know if you can disable the touchpad in the BIOS, but if you can’t, you could always unplug it. If you feel comfortable taking your computer apart and putting it back together, go to http://support.dell.com/support/edocs/systems/ins4000/en/sm_en/remove.htm to get instructions for replacing the palmrest assembly. Instead of replacing the palmrest assembly, though, just put the old one back on, but don’t plug it in.
That’s way easier said than done. I don’t have the nerve to open the laptop up and tinker with its innards and wires. There’s a computer maven in my mobile-home park who salvaged the 1-gb hard drive from my old Compaq 4000; it’s now hooked up to the laptop so I use it as an E: drive. I might persuade him to disable the touchpad since I haven’t the foggiest notion of how to do so myself.
Shit, it’s a Dell. Every Dell laptop I ever owned had this problem and it’s a hardware problem. I’ve probably had this replaced 10 times in my Dell laptop career.
dell’s are good value. But not designed for someone who rests their hands on the laptop, and they really aren’t built for the road warrior business trip guy.
With my Dell, the touchpad can only be disabled in the BIOS. This can be accomplished by holding down a specified key when the computer first boots. I can’t remember which key and I don’t have access to my laptop, but the initial boot screen should tell you (e.g. “press the X key to enter setup”). I beleive it’s either F2 or DEL.
You’ll see the BIOS menu, which is a simple text-only menu system that you must navigate using the keyboard. If you search around for awhile, you’ll find a way to disable the touchpad. Save and exit, and the next time you boot, your touchpad should be disabled. Be careful not to change any other settings though.
Well, if you ever saw me you’d probably conclude that I rested my hands on it and thus caused the drifting cursor. In fact I use an exterior keyboard and mouse. And with my big hands, fingers and wrists I hardly ever touch the laptop itself, except to switch it on.