Trying the simplest things first:
1. If you haven't run Scandisk and defrag, boot into Safe Mode and run them both (Start>Programs>Accessories>System Tools).
2. Boot into Normal Mode; see if the problem is still there
3. If so, shut down everything running in the background...most of which will give itself away with an icon in the system tray and can be shut down through a right click on the icon(s). (If that solves the problem, then you've just got too much stuff running (unlikely on your machine but it can happen) and you're going to have to stop at least some of it from loading on startup.)
4. For the download problem, empty the browser's cache and also run Disk Cleanup (Start>Programs>Accessories>System Tools, dump everything possible).
5. If that clears up the download problem, get, install and run
Spybot.
If none of that works, post back.