I Need Anti-Virus

Arghhh, there's some stupid virus on my computer, whenever I use the internet, after about a half an hour, the task bar and Start button disappears, reappears three seconds later with all my windows rearranged in different order. Then, two hours later, the screen will flicker and suddenly it will say that I'm offline, but my internet is still connected and it won't let me disconnect so I can start over. So I have to restart my computer. I tried downloading an anti-virus software this morning and it won't install, I don't know whether its my computer or the crappy software. I would dish out seventy dollars to get Norton Anti-virus, but I'm flat broke. I spent so much money in the past month just going out and shopping. Looks like I'm going to have to beg my parents for money or ask my friends to lend me their anti-virus software. Neither option sounds very appealing at all. At the moment, I really hate my computer and the internet.