Based on the description of the problem, it may well be that the problem you are encountering is specific to the script, so we should explore that possibility (and potential workarounds) before writing this off as an OS/browser compatibility problem. Inquisit 2 was tested with Vista, and I would expect that if there are compatibility issues with Windows 7, they would be limited to specific features. Anyway, the fact that you are running Windows XP suggests the OS isn't the issue.
Can you reply (or email support at millisecond dot com) with the script that is causing problems?
Also, what browser and version are you using? I'm not aware of any incompatibilities with IE8 or Firefox 3. However, IE 9 and Firefox 4 were very recently released, so it's possible some new issues have popped that we haven't yet heard about.