Malware has been a problem on computers for years. What started as problems with a few viruses is now an international problem with viruses, worms, trojans, botnets and more. The most important thing you can do for your computer is put a good anti-malware suite on it, and there are several available.
On Monday I had problems with one of our remote computers because a file was not found that was needed to run some of the programs. I spent time and reloaded the operating system. A few hours later that computer had the same problem again. Then another computer had the same problem. It turned out that our anti-virus was too aggressive and had flagged that file as a trojan and therefore had quarantined it. Once I realized this I reloaded the file and then modified the anti-virus scanner to accept that as a valid file. Problem solved. But I know the program was working.
Make sure your anti-malware is working. It should be updating at least every day and scanning your system regularly. Your computer is probably being attacked several times daily so it needs good protection.