I would say your problem.
I have similar issues when running Google chat, Vent, and WoW all at once. Specifically, google chat and Vent don't like each other too well and will try to fight for control.
There are many programs out there that don't work well when running other programs at the same time. PS can't possibly try to be fully compatible with them all.
I'm not familiar with the XFire program, but I'd suggest looking through it's options.
Are you running PS in full window mode? If so, XFire may be set up to try and take focus and pop up as soon as you recieve a message. This action can cause problems for many different games, not just PS. I'd suggest try to change XFire's options around, or run PS in a windowed mode. That way, when it does pop up, the OS doesn't try to minimise PS, rather it just brings the XFire window up in front of it. Should reduce problems.
Edit, reworded a few things for better clarity.
