Dell virus?

I have a dell computer that I got for christmas about a year ago.
it's worked pretty good until about 2 months ago, now it runs soo crappy slow.
it came with internet explorer 7, but I deleted it bc like a bunch of pages would come up at the same...

time and cause my computer to freeze, I now use firefox.
even tho I deleted it, a pop up still comes up every now and then.
I also downloaded Norton AntiVirus free trial bc my computer still hasn't been working right.
it goes slow and doesn't load half the pages of websites I go to (I don't have limewire, if that makes any difference)
this norton thing has said it's trapped this Trojan Vundo or something and destroyed it.
then a few days later a list of a bunch of the trojan things were listed...
what should I do?

yeah I pretty much have everything backed up, music on CDs and pictures on photobucket.

I have a dell to I got I think 2 years ago for christmas
and its been nothing but trouble and a mess and really frustrating!
take it to a computer place to get it fixed thats what I recently just did and now its running pretty well so far.

I've had it at a computer place before, over the summer my hard drive crashed or whatever.