if you'd rather not download anything else then your stuck with it then.
Antivirus2008 doesn't just install it's self into program files, it puts several files in the windows directory and several entries into the registry. If you start trying to delete these without knowing exactly what your doing then you will end up with a non working computer.
the free version of adaware will get rid of it and it is a good idea to have some anti malware/spyware software on your machine.
http://www.download.com/Ad-Aware-2008/3000-802 2_4-10045910.html?part=dl-ad-aware&subj=dl&tag =top5&cdlPid=10903602
But if you don't want to download anything else then it was pointless you asking the question in the first place, because all the answers are going to tell you to download and install anti malware software to combat malware installed on your PC