I am very wary of paying for anything that makes a diagnosis and then recommends itself as the cure.
There are lots of free options out there that will keep your PC running fine.
Some are even inbuilt into the system:
If you have the XP CD put into the drive, cancel the autorun and go to:
This will cure a lot of XP "errors".
Get CCleaner from click here and use it to clean the registry and remove any temp files that have built up over time.