If the computer is behaving oddly or you're getting strange popups and messages even when you're not using your browser that can be signs of an infection.
But the only way to really determine if you got an infection is to scan your computer with an anti virus program.
You should install an Anti virus program if you don't got one.
AVG is pretty good and it's free.
You should know that no one anti virus program can detect all viruses though. Therefore it is a good idea to get a second opinion, from a different scanner if you suspect an infection. You can use an online scanner for this purpose. These doesn't replace having AV software installed though.
Here are links to a couple of good ones:
Kaspersky Online ScannerF-Secure Online Scanner
