why do you want to reinstall?
option 1.A repair will just replace windows key files and remove your drivers so you`ll need to reinstall them all again , it will keep files intact but obviously if its clogged full of crap it will still run slow.
option 2.To reinstall windows completely put the XP cd in go into the BIOS choose to boot from cd/dvd rom. Press enter when prompted and go through the installation process format c: and then choose to install windows there.
Take note that option 2 you will lose all your files so best option is to buy some dvds or cds and get backing up your data. As for wifi do what snypercore said. If worst comes to worst you have to reset your router and re enable all the security. You will not have to reinstall the router drivers or software! just your wireless adaptor drivers which should be easy to find. Best option is to download all the drivers you need before reinstalling put them on cd and then it will be easy to get your pc going again once windows is running.