If a genuine copy of XP Home was installed 5 years ago it will have come with a "sticker" with your COA (Certificate of Authenticity) code on it. Often this is stuck on the case of your computer somehwere.
The sticker will look a bit like this
http://www.telecommander.com/pics/links/operat ingsystems/mswinxppropeeloem/mswinx2.jpg
There will be a series of numbers on it like:
12345-ABCDE-67890-FGHJK-TYUI9
These numbers are YOUR license for XP.
The person who reinstalled XP on your computer should have used THOSE numbers to reinstall it (he may have done if he could have got your CD to work).
As you have already paid for XP Home once you should not need to pay for it again.
So what you need is the MEDIA (the CD). If yours does not work you can pick one of those up (without the COA of course) for a few pounds.
(Did the guy who repaired your PC give you the Windows XP CD back?)
If you HAVE got the COA sticker to hand I believe you can convert a pirated copy of XP to a genuine copy by running a program and putting in the genuine code.
Maybe you need to phone Microsoft and see if you can do that.