Windows

If virus scan the system with a virus scanner,you may want to include using a number of spyware utilities to scan system

Depending on how you reached your conclusion that was a virus may not be that,XP has a built in feature to reboot when a hardware or soft ware problem is detected upon boot

Use Safe Mode to check Device Mananger for any conflicts or yellow exclamation marks besides hardware.

This could be either hardware or software. Boot into Safe Mode (F and post a comment to let us know if it runs longer than 2 minutes.

If it does run longer in Safe Mode, scan with AV and Spyware checkers (be sure they are up-to-date) before you try to boot in normal mode and post a comment to let us know what happened.

This is endemic where the harddrive medium has been used up and is no longer even in the machine, it is also, less common, a symptom of an antenna that is gone or no longer working correctly.

I would boot into safe mode and disable anyhting that is starting up. See if that helps. If it is the virus scanner doing that (because you said it is rebooting to fix a virus problem), disbaling it should stop it.
Here is my personal list of free scanning.

As of 4/23/05

To completely avoid spyware and viruses, turn PC off. Truthfully.
If you really want to use it, prevention with running programs are best. Read their docs on how to use them and update them. All these are FREE.
Oh, yes! KEEP WINDOWS UP TO DATE! Use the AUTOMATIC UPDATE feature.

ONLINE VIRUS CHECKS, no software install required other than an Active X component if required (some do removals):
Symantec Security Check
http://security.symantec.com/sscv6/home.asp?langid=ie&venid=sym&plfid=23&pkj=QQWRORVWHFHMFNZMBBX
BitDefender
http://www.bitdefender.com/scan/licence.php
Panda ActiveScan
http://www.pandasoftware.com/activescan/com/activescan_principal.htm
Trend Micro Online Virus Scan
http://housecall.antivirus.com/housecall/start_corp.asp
Trend Micro Housecall
http://housecall.trendmicro.com/
Virus Threat Center Blog
http://virusthreatcenter.com/?tag=zd.ft.fs.vtc
McAfee Virus Removal Tools
http://us.mcafee.com/virusInfo/default.asp?id=vrt

ONLINE SPYWARE CHECKS
ZoneAlarm Spyware Check
http://download.zonelabs.com/bin/promotions/spywaredetector/index3.html

REFERANCE
http://www.download.com/spyware-center/2001-2023-0.html?tag=note

ANTIVIRUS SOFTWARE (again all FREE)
Free avast! 4 Home Edition
http://www.asw.cz/eng/avast_4_home.html
AVG FREE
http://free.grisoft.com/freeweb.php
AntiVir? PersonalEdition Classic
http://www.free-av.com/index.htm
BitDefender Free Edition v7
http://www.bitdefender.com/bd/site/products.php?p_id=24