If you say it was a virus, just reinstall windows again. Or if it did not total muck it up, you can scan it in safe mode with system restore off, with GRISOFT AVG (http://www.grisoft.com/us/us_index.php
) this is free.
But if you wish to install a new HDD, you will need to determinate the correct jumper settings, detect it in your bios and then install your OS.
As for if you need to reinstall your OS at any time, ME pretty much will suck on your current system. My advice would be try to get a hold of win98 from a friend or buy it. No one in this forum will burn a copy for you, it is illegal.
As for old HDD's, i dont have any. I found it hard to find a twenty GB new, try ebay. Or go to a fleabit computer shop where all their stock is 10years old, lol.