Software changes

I'm currently using XP Pro with sp3 and all current updates. I'm also using Mozzilla and Thunderbird as I was told they were safer with regards to infections,adware,etc. I.E.8 is also on the system somewhere although it will no longer activate. It appears that during one of the updates some parts were changed and it may be possible I have some malware and my knowledge of computers is extremely limited and each of these changes makes it difficult to maintain any working adequecy.
Any possible help to narrow this would be greatly appreciated.I'm not sure if this description is defined enough, please advise.
Skynard