No dont worry about that,
Re-installing only deletes the current files it originally downloaded, and replaces them with the same files.
First of all, vista sucks.
Get a cracked windows 7 or XP
What I did was check processes, make sure nothing over 30k is running, that isnt needed.
Use a game booster too, that helps alot.
If all else fails, save up for a new computer? ;P