New XP install is not clearing old stuff

For the last fortnight my old Dell GX260 has been getting slower and slower - and locking up. First up, I blamed too much stuff on it, so I thinned it right down. Still no good. Random things were going crazy, like my cursor suddenly going nuts, or the right click menu causing instant freeze, with massive CPU usage. I tried Avira and a few other good malware scanners, but apart from a few viruses the Avira found, no solid result to fix the problem. So I went for a new XP install.
But when I downloaded some favourites, up came the 'already installed' message. WHAAAAT ??? Can't be. So another install - with the same result.
I was apparently wrong in thinking with a new OS install, all the old stuff was swept away, because this is not happening. In addition, the thumbdrive I used to shift some data seems to have corrupted my laptop as well. Same symptoms - freezup, high CPU usage, dead slow. Again, scans reveal not a thing wrong. So can someone fill me in what to do?

 

when you install a windows OS do a LONG format and nothing should remain
sounds like you either repaired an install or did a quick one a long will get rid of all info and the MBR as long as your booting from a cd
and if for some freak of nature that does not do it then use some thing like partition magic make a boot cd and wipe the drive from their then boot the windows disk