There are several possibilities here.
1) SP2 is known to have a long list of incompatible programs, which may be conflicting with it and causing the restart.
2) You only have 128mb of RAM. Windows XP probably eats away most of that memory alone, and running other programs may totally eat away the system resources, which caused the restart to prevent the system from becoming unstable. Type "dxdiag" in run and check how much memory is being used,
3) Spyware/Adware may be the culprit, scan with spybot/ad aware and delete any files found.
4) You may have a dying power supply.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w