A computer worm is a noxious self-replicating program. Compared to other infections, a worm does not have to enter a program to multiply and go from computer to computer.
Worms are regularly used to make a secondary passage for a programmer to access the victims PC, for use in a botnet, to send spam mail, or to anonymize their own program history.
Similarly as with all malware, a slow Internet connection without any reason is a certain indication of a worm assault. Receiving a lot of unwanted emails, more than just the regular spam we are all accustomed to is another sign of a possible infection. Strange browser behavior or warning signs from your anti-virus and firewall systems may also occur.
If we have all reasons to suspect a computer infection we don't need to panic. Many viruses and other types of malware are designed simply to cause chaos and it may be the same case.
Supposing that you have an anti-virus program installed
Most probably a pop up message appears on your desktop from time to time or you have a shortcut on there. Simply open it and run a quick scan. Most of the time it takes from 10 to 20 minutes to finish depending on your computer's specs and how many files you have stored on your hard-disk.
If nothing shows up after the scan is complete and your computer behaviour didn't improved you may want to make an indepth scan. But this usually takes from 2 to 3 hours or even more! So you will have to be ready to do this in the morning and move to other things. I advise against leaving the computer overnight.
If the scan was completed, nothing was found and you still have all the reasons to beleive that your computer is infected you may want to change your anti-virus.