How did it get this big and how do I change it?
I assume you’re using MSIE.
Click tools, Internet Options. Under the general tab - temporary internet files, click “Delete Files”. Then click Settings and you can adjust the size of your cache.
It got that way by surfing the internet. Your browser stores pages and images on the disk so it will be faster the next time you visit a given webpage.
The following assumes you are using IE as your browser:
In IE…click Tools > Internet options > General Tab. Click the Settings button under Temporary Files. Set the slider for whatever cache sise you want to use. Click OK. You can now clear the history, if you like. This should put things back to normal, unless you got something funky running in the background to change things like this.
That’s not it, the cache is 218 Mb.
Have you got a lot of offline content?
Build an NTFS boot disk, and go in and manually delete the offending file.
He might not need that. If he upgraded to XP from, say, 98 or ME and didn’t convert the drive to NTFS, he’s probably got a FAT32 volume. Of course, that would suck, as NTFS is vastly superior. 
I remember reading a document that outlined all of the information the operating system stores on your computer. I seem to have lost though, so this is from a very vague memory as I really only scanned the document. The main thing I remember is that the OS records and stores information about every file on your system and kinda hides it. This could be the runaway index.dat file. I wish I could be more helpful.
You can also turn off the indexing that Greymot is talking about. You have to get to the “services” controls somewhere under the control panel (I don’t remember where it is), then set the file indexing service to the off state so it doesn’t get started at all.
It was a clean install onto an NTFS disc partitioned into:
c:windows (system)
d:data
m:memory (pagefile)
p:programs
t:temp
The offending file is:
c:documents and settings/craig/local settings/history/index.dat
It isn’t visible there even though I have the setting set to show hidden files and to not hide protected operating system files.
c_goat: are you referring to the System Volume Information? I have that folder on t:.
QED: I have a dozen pages saved for offline use.
If you can’t seem to get the file size down, just open the file with notepad, select everything except the file header, that should read something like - "Client UrlCache MMF Ver 5.2 ", and just delete all the text in the file. Then just save the file back into its’ original place overwriting the original file.
A bit of a pain in the arse, I know, but it does work.
Hope that helps
Achilles I highly doubt that would work. Opening a 4GB text file requires nearly 4GB of page file which, if he has available (unlikely!), will only expound on the problem.
He could always open the file, copy out the header into a new blank text file, and save that text file over the old index.dat.
Where there’s a will…
You cant just delete or modify the index.dat files. They mostly contain URL keystrokes eg every URL you have every typed into the IE address box.
The easiest way to delete this file, (since it is protected) is to log in as an admin and then delete the file.
You have to reboot and log in as admin, since if you just switch users then the original user still locks the file.
Th best solution is not to use IE…
That’s not true. You can modify the files no problem - even if your not an admin. Just as I described above.
Can’t argue with that ! 
Uh, dudes? I can’t see the file. I only encounter it when I try to defrag.
I’m curious if XP’s handy “Disk Cleanup” utility would help get rid of this file. Purhaps worth a shot? (in the dark that is)
Open any explorer window (ie, double-click on My Computer), go to tools–>Folder Options. Click on the View tab. Make sure the dot in the radio button for “Hidden Files and Folders” is next to “Show Hidden Files and Folders” and the “Hide Protected Operating System files (Recommended)” is UNchecked - Windows’ll squawk at you, just tell it you want to go ahead and do that. That should make sure ALL “hidden” files are showing. (I would also uncheck “Hide file extensions for known file types”, but that’s just me )
critter42
Flander: Disc cleanup offers me a freeing up of 2,271,090 KB, mostly through compression of old files.
critter42: I’ve done all that. Including don’t hide file extensions. Bloody Windows.
It is a truly hidden file, along with a bunch of other ones, that stay hidden even when you select show hidden and system files.
If you know the exact path and file names to these ‘super hidden’ information gatherers, you can get to them usin your command prompt.
Good Luck!