I just tried to cut and paste a relevant post of mine from another site, but this system wouldn't take it - it's too long.
Here's an excerpt from that post that may help:
REBOOT INSTEAD OF SHUTDOWN
Most Win XP shutdown problems reported thus far have been that it reboots when shutdown is attempted. This may be a global symptom emerging from several distinct causes, because, by default, XP executes an automatic restart in the event of a system failure. Therefore, more or less anything compromising the operating system during the shutdown process could force this reboot.
Disabling the “restart on system failure” feature may permit the exact cause to be isolated: Right-click on My Computer, click Properties, click the Advanced tab. Under “Startup & Recovery,” click Settings. Under “System Failure,” uncheck the box in front of “System reboot.”
Here are some things that have produced this reboot-instead-of-shutdown symptom:
By now, the Roxio/Adeptec Easy CD / Direct CD software is well documented as being the major cause of this undesirable shutdown behavior. SOLUTION: Roxio has released new drivers (here) to solve this problem in both the Platinum and Basic editions of Easy CD Creator 5. As expected, at least half of the Win XP shutdown problems went away with the release of these patches.
One warning about this patch comes from correspondent Bert Smith: Be sure to read the directions! “Roxio Easy CD Creator Platinum 5.0 can be a real hassle to get working under Win XP,” Bert wrote, “and there is the risk of your computer not booting if you blindly go ahead and install it without first consulting the Roxio Web site.” Bert also mentioned that Roxio’s “Take Two” backup program (normally part of Easy CD Creator 5 Platinum) is uninstalled when the Roxio patch is applied.
Direct CD. Many Easy CD users (but not all) found that installing Easy CD 5.0 does not cause the shutdown problem, provided they do not install the Direct CD component.
UDFRINST. Several people solved this reboot-on-shutdown problem by deleting the UDFRINST file. This file is part of the Roxio CD-RW software for systems not using Direct CD.
CDRALW2K.SYS. Correspondent Larry Blumette identified the CDRALW2K.SYS file (version 1.0.0.1048) as the Roxio file causing his shutdown problems and error conditions. When he deleted or renamed this one file, his problems went away. (Of course, you lose your CD functionality that way, too.)
Whether or not APM is enabled makes a difference — but the effect could go two ways. Some users report that XP reboots on shutdown if APM is enabled, but shuts Windows down just fine if APM is disabled. Other users report exactly the opposite behavior. According to Jack Dunne, this is similar to a known Windows 2000 problem. The issue seems related to the computer’s specific hardware or BIOS — so, as with all NT operating systems, stick to the Hardware Compatibility List where possible.
Y-SB3 Logitech Internet Keyboard can also cause this problem. If you use it as a simple generic keyboard, there’s no problem; but, if you install the Key Commander software that drives the special Internet functions, Win XP will restart instead of shut down. Unfortunately, Logitech has decided that they will not be updating this driver for this keyboard. (Tip from Jan K. Haak.)
Logitech MouseWare 8.6. Windows reboots when shutdown is attempted. The software caused a BSOD with KBDCLASS.SYS. Removing the software solved the BSOD the problem. (Tip from Pablo Cheng.)