Solve this problem

Purchase a copy. You will be legal, and you'll have less problems in the long run!

Well, in order to get a genuine copy of Windows, it is required that you purchase it, it is not possible to to obtain legal copies without paying. It is possible to obtain an illegal copy of Windows XP online without having to pay for it, but note that it is illegal and you should pay for this software, a lot of hard work is put into making software.
But being that it is illegal, commonly your ISP and / or the company that owns the software will hire people just to monitor the internet traffic of others and anyone logged downloading files such as that will be liable to jail time and a monetary fine worth well more than the software you stole.

But, if you don't feel like heeding to my warnings. A simple google search of :
"Microsoft XP download tpb" will get the job done.

Your computer should have a key on a sticker on the side of it. If you have taken care of that sticker, and you can read that key, then that is the legal copy of windows that you own for that machine. If it says 2000 XP XPPRO or VISTA etc, whatever, that is the version of windows that you already own on that machine. Your computer manufacturer should be able to provide you with a copy of windows for that machine. If not, then there is good news, and bad news. GOOD news is, you can purchase an IDENTICAL COPY of that version of windows and place it in your computer AS WELL AS ANOTHER computer (because you have the old key on the computer case and the NEW key on the disc). Bad news is, youd have to drop some cash to buy the disc.
There is ONE LAST OPTION: Take your computer to a pc shop. They have several versions of Windows available and can restore your windows with drivers to a generic condition, and since they would use your key on your machine, it would be legal. It wont be free though, but anything worth having is going to cost.