Guys this is VERY IMPORTANT. I have a computer. It came with Windows XP Evaluation Version as I didn't want Windows Vista. Then the 30 days passed and it didn't work, so I bought Vista Home Premium. Vista worked nicely until some viruses got in and it got messed up. So a friend was offerig me his Windows XP Professional for 10 $ so I bought it. I installed XP again but the funny thing is that it didn't recognize either my video card or my sound card. So I bought Windows Vista Ultimate to get out of any trouble and stop spending money. I installed it, but today I tried to install XP (not professional, one that I borrowed from my friend). I installed at D Hard Disk, but now that Vista is in C, only XP works. Now here is the real problem. I tried to re-install Vista, but when I put my CD it says "This file may be corrupted or damaged. The header checksum des not match the computer checksum". What do I doo?