Slow Pc - Upgrade Memory - OLD CRAPPY Settings - Svchost.exe's

I NEED urgent help, not THIS computer but the person I live with's computer is EXTREMELY slow, things take up to 10-20 minutes to open sometimes, it takes at least 15 minutes to start up or shut down, everything lags, we've tried EVERYTHING, virus scanning, spyware, malware, cleaning the registry, defraging the registry and harddrive, cleaning up, EVERYTHING, anytime you shut down after applying some sort of setting it says it's saving the settings and then you start it back up and it acts like it never saved it, it goes back to the OLD CRAPPY settings, puts every error back on that the program found, recently the boot.ini has become corrupt or invalid, there are two hard drives with plenty of space and we've had this problem for a couple of years! We can't figure out what to do, we both have the same computer, bought it at the same time, mine runs like a charm but their's does not at all, the cpu usage is sky high when you aren't doing anything and sometimes it is 0% when you ARE doing stuff. There is about 10 or more svchost.exe's running, two under a seperate username and none of which we can stop. There are diagnostic lights on the back BUT anytime there is anything wrong with the lights we look in the manual and on the offical website and NO ONE HAS THE LIGHT COMBONATION THAT WE DO!! It's a Dell Dimension 2400, right now it has both 512MB and a 128MB RAM memory stick, we can't upgrade the memory or it fails and starts beeping, Windows XP Home Edition,