Actually, if you no longer use the old PC or the PC that the licence came off, then you can activate it just fine.
Many people will tell you that this is still not legal but MS will still activate it just fine, even if you have to phone them up they will still do it no worries... As long as its only on one PC.
Sure, the basic rule is that its for the PC you bought before, but you can change any hardware you like and even get to the point that nothing from the original PC is the same, but it is still the same PC... Until you change the Motherboard of course.
This is a stupid arguement however and its really only arguing over the most retarded issues.
Just use XP if you are heppy with it, and you will have little hassles activating it with MS as long as you tell them that you have changed a few bits but its still the same PC then you are ok!
As for the XP vs Vista question, this is realyl only down to personal taste.
XP has proven its stability now year upon year and it is flawless.
Vista on the other hand is now mature enough to have proven itself and indeed it very much has, so the stability arguement is one that no one actually cares about anymore.
XP will run pretty much 100% of what you want it to, while Vista still have a few issues here and there, but I think you will find that 97% of Vista users wont know what software does not work on Vista.
If you do go for Vista, please FFS dotn go with 32Bit
