Do Need To Reinstall OS / Possibly A Corrupt Registry?

I have very minimal knowledge of computers, so please bear with me. I'd appreciate any help or advice. I'm not really sure what is relevant and what is not, so I may be including information that is completely useless. Until this weekend, I had xp home on my home computer. I use a program at work that I need to be able to use at home. I was using it without any problems until about 2 months ago, when the program did an upgrade. Since then, I have not been able to run this program at home. I have spent numerous hours in an open meeting with them, trying to fix the problem, and it seems to have baffled a lot of their tech support people. Finally, one of the tech support people noticed that I had xp home as my OS, and then told me that the program did not support xp home, only xp professional, so she couldn't help me any more, and said she suspected that was the problem.

I didn't think this was really the problem, but given that they would not help me any further, I purchased an upgrade to windows xp pro this weekend and installed it without any difficulty. Not surprisingly, the program from work still doesn't work. I spent another several hours earlier this week in an open meeting with the tech support people. The head guy told me that he thinks that there is a problem with my registry, and that is why the program is not working. I am not quite sure how he came to that conclusion, but it seems to be related to the fact that this weekend, after I installed xp pro, that one of my automatic updates (for windows installer 3.1 ) did not install with the error message 0x80070005. I have never had a problem with other updates not installing before, at least not that I'm aware of. Apparently, this update is crucial to running the program from work - or just updates in general are crucial?