ok so 40GB is small by todays standards. but its sufficient for XP to run and a browser to function.
Lets assume for the moment that its therefore unrelated to firefox, and if you free space, it will be eaten up fairly quickly by something else. so whatever diagnosis you do, you have to act fairly quickly.... Its also a case of yet again going over old steps.
for starters load XP in "safe mode with networking" (keep hitting f8 after you turn it on should bring up the boot menu with the option shown)
Then run a disk cleanup again. (Start->All programs->Accessories->System Tools->disk cleanup) Clear out what you can in the "Disk cleanup" Tab.
If you are still out of space after this, please can you then confirm the size of these folders on your C: drive (rightclick->properties) so we can establish if you can free space in places.
C:Windows_Temp
C:Documents and settings_"your username"
C:Documents and settings
C:Documents and Settings_"your username"_Local Settings_Temp
(torn forum wont allow a backslash in the post, so I've put a _ instead.)
If you don't know the path of your user profile folder, then open a command prompt and type "Set U" (without the quotes) and press enter
Please also confirm how many user profiles folders are listed in C:Documents and settings just in case there are more profiles eating up space, including any "Temp" profiles.
Can you also give me an idea of what programs you have installed, just so we don't miss anything else obvious!
Lastly if you free enough space, I recommend rebooting normally and running (again if needed)
https://www.malwarebytes.org/mwb-download/ MalwareBytes antimalware (free)
Update it, Do a custom scan and include all clean-up options.
http://www.eset.com/us/online-scanner/Same as- update- include PUAs etc in the scan options.
For the moment I won't suggest turning off the pagefile, as if the space gets used up, you wont be able to turn it back on again...
If you are unsure of what you find, let me know before deletion, your PC is (by the sounds of it) close to not working..
got to go, Real life calls...I will check back again soon.
MC.