Depending on how badly infected your system is will determine how long and if your system can be cleaned up effectively. The authors of viruses, spyware and malware are deploying more and more sophisticated techniques in their attempts to attack your computer.
I use a number of software tools which largely run unattended to clean up and remove instances of viruses, spyware and other malware.
If you make the effort in the first place to minimise your vunerabilty and therefore minimise the risk of such attacks you will be spared the pain of either fixing the problem yourself or paying a professional to do it.