Reinstalling xp

i have a dell optiplex 745 and i wanted to reinstall windows xp pro. i got a windows disk the other day with the product key but if i use it to repair my windows will i be using one of my installs???

 

I am not sure i fully understand what it is you intend to do (repair or re-install)
If you want to re-install all you have to do is put the disk in and re-boot the PC it will then boot from the disk and at the appropriate time insert the key.
If the disk is genuine MS you will not have any problems.
the same for repair just insert the disk and re-boot and select option repair.

 

i was told tghe key is only good 3 times and i put it on a computer i just built and i wanna start from scratch on this comp so do i hit repair or install???i already burned the files and things i wanted onto dvd so now what?

 

As far as i know there may be a limit to the number of PCs you can use it on but on the same PC it is normally unlimited?