CPU starts constantly spiking in XP
I don't see any suspicious processes in the Hijackthis log. Try downloading and running Driver Sweeper, selecting all the Nvidia stuff, then after cleaning all the nVidia stuff re-boot, install the latest nVidia drivers.
I don't see suspicious processes either, but they say any process could be safe or suspicious, so you've got to start with the unnecessary and most likely to be suspicious. Those I would investigate and possibly uninstall are: mDNSResponder.exe and STacSV.exe... other processes are Apple and Google applications that are basically useless... you may want to remove them specifically because they don't serve a usefull purpose in my book. I also see a few toolbars that should be removed. Toolbars started out as spyware and by now they have been mostly accepted but I wouldn't trust any other than the Google toolbar.
What is mDNSResponder.exe / Bonjour and How Can I Uninstall or Remove It?
What is stacsv.exe doing on my computer?
One thing I've noticed is that some processes that consume high CPU %, go down immediately as you open the taskmanager, when I find this problem, I keep the taskmanager open so I can catch them in the act.
Check the Bios settings to see if the Processor Core is disabled and can be reset... if you don't find an option to enable it or if it's not disabled, just select to set BIOS to default settings.
You should also; Reset the security registry and files permissions to default running Using SECEDIT.EXE, and check the Plug and Play and SSDP Discovery Service are enabled,.. before attempting to update the Graphics drivers.
Also disable the Remote Assistance and related Services, as well as the Computer Browser, Server, Remote Registry, Telephony and Telnet services to secure the computer from external access... and finally I'd suggest you scan for Rootkits, disable startup programs, delete temp files, Internet temp files and junk files.