It's hard to guess what your problem might be without more information, but I'm running a desktop PC and a Dell laptop, both with XP Pro SP3. I have no problem using IE8 or Firefox on either machine.
Did the FiOS installer install any software on your PC? If so, I suggest you uninstall it. It's not necessary.
Where are you located? What kind of pings do you get to a server like dslreports.com?
Does the problem you're describing happen when accessing particular websites, or all of them?
What speeds do you see at the FiOS speed test: http://my.verizon.com/micro/speedtest/broadband/
and what speeds and pings do you get at speedtest.net?
For starters. 🙂