Do you mean you have formatted 25 times and installed WinXP?
You can format your system as much as you wish and you can still activate Windows. You should be able to activate it over the internet as usual, but Microsoft will stop you from doing this if you keep formatting it, so that you will phone them and they will ask you why you keep formatting. If the reason is genuine, then they will give you a code to type in and allow you to activate Windows over the net again. (They do this for security as someone may be installing Windows onto identical computers and teh activation tool wont know this if they have the same hardware.)