if you re install windows you will need to get all the updates again im afraid. re installing windows would be my last option to try if everything else fails.
how much free space do you ave on your hard drive and have you run a disc cleanup and defrag lately? if not try this after uninstalling any software you dont use anymore to free up some space if running low.
do you have an up to date antivirus and have you run a scan with this lately? maybe also try running a check for adware or spyware with adaware and spybot as this can sometimes slow things to a crawl click here click here
can you post your specs please may help.