When I bought my computer it had all programs installed. Now it wont download service pack 3 and I dont have a validation key. What can I do other than purchasing a new copy?

Ignore SP3 - so far it seems a load of trouble anyway. You possibly bought a pirated copy of XP, so will not be able to download any MS patches, let alone SP3, so live with it.

K thanks. Could it be that because it wont upgrade, other things are giving me problems? For example lodgeing electronically using a program which is not windows?

You should not expect major problems if it was a valid install in the first place. Continue to use the Microsoft Windows Update.
Your problem will show up when that hard drive fails.
Microsoft now has the capability to tell if you are using an unauthorized copy, but they likely will not shut you down, though they may tell you your copy is invalid and forbid further downloads from Microsoft.
You can retrieve your Windows Product ID, by downloading and running the Belarc Advisor www.belarc.com or the Everest equipment audit. Both are free.
During their checkups they will tell you what the Windows key is. You can then print out that report.
Where you might have problems is if you have other Microsoft software installs on that machine.

Thank you RayBay. Is it safe to run the items you referred to?

Maz:-)

Belarc advisor is perfectly safe to use. Raybay is right, you need to know pretty soon if (a) you have a valid OS and (b) if you have failing hardware.

Yes. Safe and in common use. Recommended frequently on this forum. In my experience on hundreds and hundreds of laptop and desktop computers of varying ages, totally safe. Better yet, truly useful.

Definately not, because of this:

You must be concerned about any program that may "call home" (ie checks Windows updates)

Is this Windows version legitamit?