I set a password for my computer using windows xp professional but somehow someone bypassed the first screen when the computer boots up and you select the account. I disabled everything but the administratot one. Also, I have set it to input a password when the screensaver goes up. Is it still vulnerable and how do I secure my computer once and for all?
Most computers have a password protect option in the BIOS/CMOS, and won’t run anything without the password, if it’s enabled.
For a bit of overkill, my case’s front door and side panels lock, preventing anyone from opening the case to reset the BIOS or, for that matter, turning on the computer at all.
BIOS password is best but that can be defeated by a knowledgable geek if he has a few minutes and can open the tower.
Locking tower with a key is also good.
Best is ‘mobile trays’. Take them out and take them with you. That you way you have the hard drives with you so…
Better yet, don’t put nuttin secret on your machine and when the kids or coworkers or MIL or asshat gets in your computer without permission, and against instructions, shot them. 
Alietta, to answer the other part of your question, yes, the computer is still vulnerable. A boot disk with DOS2NTFS will allow anybody to access your files.
The BIOS method will prevent somebody from even booting a floppy without the password. As others have mentioned, it’s possible to reset the BIOS by removing power and shorting a jumper on the motherboard. You would know there had been an intrusion however because your password would be gone.
Another solution would be a PGP encrypted partition housing all your sensitive data.
But unless the back side is also locked up, you can just pull the power cord out & re-insert it. This will restart almost all systems I’ve worked with, and then you can jump into the BIOS setup during the power-up process.
There is no possible way to completely secure your computer if other people has physical access to it, full stop. So the first step is to stop people from accessing it. Once you do that, others can only access it remotely. You can usually prevent intrusion by using a firewall - configured properly, of course.
As stated aboved, an encryped partition would protect data even if the harddrive were stolen and placed in another computer. PGPdisk is free and effective. Here is an article about it. Here are bunch more hard drive encryption solutions.
You’ll still need to reset the BIOS password first (method depends on the model of computer) before you can get into the BIOS setup.
Windows XP Professional has encryption built in. From Explorer, select the files or folders that you want to protect, right click to bring up the context menu and select Properties (or type Alt+Enter to bring up the Properties dialog directly). Select Advanced, and then check the “Encrypt contents to secure data” check box. Note that this only works on NTFS volumes.
True, but this only applies to PC’s that had a BIOS password set beforehand. And that’s a very small percentage of PC’s.
Yes, but the difference is that PGP can require a passphrase every time you boot the machine before it will “mount” the encrypted folder. If somebody is able to log on to your console, using the built-in XP encryption they will have easy access to all the encrypted files.