I *believe* .msp files are Vista compatible, so your mileage may vary with XP...
You're thinking of .msu files, which are the file format of the Windows Updates for Vista.
You're thinking of .msu files, which are the file format of the Windows Updates for Vista.
To the OP: What is it you're trying to do? Are you trying to patch an existing application .msi file? Is it standalone or an application deployed via group policy?
Either way, the following link may me of interest: http://www.appdeploy.com/faq/detail.asp?id=48