PC Hangs and Reboots frequently and abruptly

running on a machine (back ground guards can conflict & cause hangs & other problems). You CAN have a second one on machine for Manual "on demand" scan as 2nd opinion BUT the Main one MUST be COMPLETELY disabled before opening 2nd one. Now a days , more common to have 1 good one (AV) & use a online AV scan for 2nd opinion. Problem can be compounded if not just AV but Security Suite used. Only 1 "software firewall" active as well.

You can have more than 1 Anti-Spy program but beware of multiple background guards again; particularly ones routing thru host files & controlling settings changes.

Please give EXACT name & version & source of "installed anti-spybot ".
Many False & Rogue copycat programs out there. Most use "sound alike names" to valid programs. You should always check product at following list before visiting site/downloading/paying for any Anti-Malware type program. You CAN become infected JUST by visiting ROGUE site. See:
http://www.spywarewarrior.com/rogue_anti-spyware.htm

Over heating condition can cause a vast variety of symptoms including those you describe. If your fans inside the computer haven't been cleaned in 1 year or more this is a MUST now before more damage happens!! Artist's type paint brush & canned air recommended. Cleanliness is next to Nirvana inside a computer.
While sides are off, aim a fan at inside while using computer as a confirmation. If crashes/errors stop or much reduced it's confirmed problem.

I consider 512MB of RAM MINIMUM for XP myself. More IS better (up to a point. If converted from earlier OS [95/98?] M.B may be the limit).

Also be cautious how many & how heavy the draw of attached USB items.
May overdraw power supply or need added "powered USB Hub" if too many.
I use 100watts per attached optical device (cameras/printers/CD/DVD/scanners/flash drives etc) as a base guide line. 450-600 watts is usually good enough. Double fan is better & quality brand pays for itself in long run.

If not comfortable inside your machine, have a shop you trust (??) give best price for clean & extra RAM.

Hoping this knocks out most of your problems. Post back with results for further help. Few more possibles available. Good Luck!

Remove the case cover and see if it gets better.

Also, it's been discussed at length about AV and firewalls. Pick one. Installing more than one has caused issues.

Bob