This sounds like it may be a virus at work. Try booting into Safe Mode and running a thorough Virus scan. This tutorial will show you how to boot into Safe Mode if needed.
I would boot back into Safe Mode and do a Repair Install. This will allow you to repair your Vista installation without losing your files, settings, and programs.
Do you have a Recovery installation DVD at least? If so, you can do a clean install to fix your problem. If not, you can contact HP and order the Vista installation DVD for your computer for only a small S&H charge.
It only makes sense to upgrade to the Vista Ultimate version if you just want or need the features in it. You can compare the feature differences here.
There is no need to pay that much. HP is suppose to provide you with a way to reinstall Vista by their contracted agreement with Microsoft. You should only have to pay a small shipping and handling fee to order a Vista Recovery DVD from HP if you do not already have one or lost it.
HP computers usually ship with a recovery partition on the installed hard drive. If you take a few minutes to consult the included literature there should be detailed instructions on how to execute the recovery process. I had to do this recently on a machine and it was a simple matter of striking a key during the boot process which loaded up the recovery console. I was then able to execute the recovery process with about five keystrokes. The whole process took about 30 minutes and was completely automated.
Don't panic. Take the time to read the included HP literature. It will explain everything very clearly.