do you have some kinda virus software installed?

are you logged on as admin or something comparable?

what edition XP? sp1? sp2?

you may want to install the WindowsInstaller-KB893803-v2-x86.exe file from M$... maybe your installer has become corrupt.

are you using the software that came with the blackberry or a version you downloaded? maybe the download was corrupt, or the media is damaged..

any of those sound good?