1.unusual behavior of your system like displaying virus infected message dialogue box(install "some antivirus name"etc)
2.unwanted ads when u are browsing internet
3.slow performance of the system
4.taking too much time to load a particular website
Solution
1.Antispyware
2.Antivirus with antispyware
3.Firewall other than windows
Major Antispywares are
1. spybot (freeware)
2.xtort
3.noadware
Antivirus with Antispyware
1.AVG8 (freeware)
Internet Security With Firewall
1.Norton Internet Security
2.Mcafee Internet Security
3. Bitdefender Total Security