I use QTR on Windows 7 without any problems. You can just rerun the install and my recollection is that it will simply overwrite any older files. No need to uninstall and reinstall.
In relation to John's question, I don't think that it will overwrite or wipe any files that you've added, provided that they don't have the same name as any files in the install (i.e. you haven't edited any installed files and kept the same file name), but I always make sure I have a backup of those files somewhere just in case.
QTR has two locations for its files - under Program Files tree and under Users. You'll only see notice this if you go looking for your files, as QTRGui will combine the two sources seamlessly. The idea is that any files you create would normally be stored under Users. My understanding is that this is done for security and permissions reasons, but it also means that your files are separate from the installed files. If you like me find that tedious, and have placed your own curves under Program Files, then I'd take a backup before installing as a precaution, but as long as you have used unique file names then I don't think you should have a problem.