I am trying to repair a Dimension 2400 desktop (with Celeron processor).
It boots to XP splash screen then reboots and repeats cycle. It will boot into MEPIS Linux (LiveCD) fine and run all day (with or without case installed).

By way of background:
It was cycling as described so I shutdown, added memory, cleaned and vacumned etc.
I booted in MEPIS Linux live CD, backed up 20GB of data to a USB drive all fine.
I booted XP CD and reloaded(actually repaired) XP; Rebooted fine, installed/updated/performed full scan on antivirus,and antispyware. Did full windows updates etc. all fine. Loaded Firefox, Openoffice etc. When I closed case it shutdown and upon reboot said message about thermal event and began cycling again.

I saw on forums various posts about Dell thermal events and I checked that the capacitors, all have the little "K" stamp on top and are all flat (as they should be). The mobo is I believe a Intel 845GV. The cpu fan is running fine and so is the case fan). The cpu fins seem to be firm. The 4 lights on the back are all green (in Linux) and same in XP until reboot. They cycle a bit during initial startup and are GGGY at he BIOS Setup screen . BIOS is version A05 which I believe is the latest. nothing seems to be overly warm

Seems like XP is too sensitive to something. Any way to adjust that? Or any other ideas?
Thanks in advance!

Have you tried putting a spare HDD in the machine (to preserve the work you have already done), and installing a fresh copy of XP on it?

If you can't duplicate the problem with a new install of XP on a new drive, I would think there is something broken in the other XP install which the Repair re-installation did not fix (which I have seen happen from time to time).

I have seen this sort of "loop booting" a couple of times on xp machines but if I remember correctly they seemed to start booting again when they got to mup.sys. I have repaired them with as little as running chdsk.
If you google mup.sys you will find all sorts of little fixes that have worked for different people, but nobody seems certain what actually causes it.
Just a suggestion-hope it helps.

boot with the CD again and use the chkdsk /p

this helped my in a similar situation.

After removing, cleaning, and regreasing the heatsink/CPU and removing the battery overnight, upon start the box still cycles to the XP splash screen then reboots. When I look at messages during boot, I see the last one is the MUP.SYS. Googling on this "Mup.sys reboot cycling" topic brings up many many accounts of this problem with various solutions. Thus my problem is not thermal related (so it seems ) but this elusive mup.sys reboot cycling issue. I tried booting into the reocvery screen and using "chkdsk /p" and later the "disable mup" both to no avail. Still cycling. What next?

Solved problem (apparently by performing a full reload - not a repair) of XP. So far so good. Never did identify a root cause.