512K ram interesting, dont you mean 512MB! or 51200K!
Sometimes its quicker to format and start again instead of trying to find the route cause. If you have loads of software installed which starts on startup, this could be your problem. Run regedit from the run menu and under the startup tab try disabling any unwanted programs.
Installing windows over the top of a original installation is not recommended.
Also you could try defag the harddrive aswell as disc cleanup within drive space.
Might help might not, but if I was in the simular situation, I would format and start again creating a empty partition on your harddrive with something like partition magic to save any files you want to keep.
Very much doubtful its a virus, a quick check with a virus scanner will determine this.
Hope this helps