I have an Apple computer that also runs XP and I use both the Mac and PC sides. Firefox is absolutely by far the fastest-and most versatile with add-ons no comparison, Chrome is also fast, but not as versatile yet. IE7 is the next fastest, but has glitches, Safari is much slower than IE7 and less versatile (Its a stripped down tabbed browser that is slow) and needs improvement. Opera is Ok
My ranking is
4) Safari and Opera Tied depending on your needs.
I don't think all the railing against IE7 is deserved based on my experience. I think safari has been pumped up way too much. It's a very nice browser an a great apple attempt to become a major browser quickly, but it's way overdue for a re-haul particularly speed, bookmarking and saving tabs. I like having only the essentials on safari, it it needs a few more features.