Virus in Windows XP affected operation of Ubuntu 11.04 on double boot system.

I have a double boot system using Windows XP and Ubuntu 11.04. For the past two weeks I have used only Ubuntu when operating my computer. Yesterday I noticed my Firefox browser had become extremely slow (while using Ubuntu) and it continued being slow this morning. So, I restarted my system and booted to Windows XP where I found my Norton Suite program advising me to update my virus protection and scan the system. I followed that advice then launched my internet browser. My Windows XP boot also runs Firefox and after completing the virus scans it ran up to the speed I normally expect from my system. Then, I restarted and went back to the Ubuntu boot where I found my system there was also running up to its normal speed. My question is this: Can I be acquiring viruses while operating from the Ubuntu boot that affects the Windows XP boot? And, will that ultimately affect the speed of my browser on the Ubuntu boot?