Spontaneous reboots are almost always a heat related hardware problem.
Here is a list of possibilities that MVP Bert Kinney compiled:
Overheating (make sure your fans are all working and clean)
Faulty RAM. Have RAM tested
Over clocking of the CPU
Less common causes:
Faulty power supply
Incorrect voltage setting on the motherboard
Improperly configured BIOS setting
Loose power connectors or cables
Fluctuations in power to the system
Athlon based computer with less than a 300W power supply
UPS with a dead battery

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Regards
Ron Badour, MS MVP W98 System
Tips: http://home.satx.rr.com/badour
Knowledge Base Info:
http://support.microsoft.com/default.aspx?pr=kbinfo
> I started having my computer reboot thought it to a virus
> so reformated hard drive same then change power supply
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> HELP!!!!!
> and thanks in advance for your help