p.c keeps re-starting

I have a Pentium 4 computer running win xp home and when I power up my
computer it will go thru the power up and then start all over again (
sometimes 10 times ). I am afraid to power my machine off in case it won't
start up next time.I have a high speed cable internet provider and half the
time I loose my settings to log onto the internet. I also have Norton
Antivirus. This started about 3 weeks ago. Once the p.c is up and running I
am o.k
could this be a trojan or virus or my power supply?

Any suggestions?

Tim.