HELP! Virus killing my computer

I purchased a dell laptop 1800 (i believe). My wife uses it to pay bills online and we discovered we now have a virus. What happens is, we log on then as soon as we try to click on a link or enter a website we get a window that says “not enough memory on your hard drive) then if you close the window you get booted off the internet.I’ve run Symantic and other disk cleaning programs on the computer and it always find stuff and kills them but sure enough, when you try to get back onto the internet the same thing happens.
I am hoping this is an easy fix and that someone here knows how to fix this problem.

Thanks in advance.