There are a lot of spyware and malware out there. If you do a lot of browsing (especially with Internet Explorer) there is a very high risk of getting infected with one of these. Also some programs contain embedded software called adware. Adware displays advertisement banners and gets annoying very soon. Spyware/malware/adware etc. use some standard techniques to get itself installed permanently on to your sytem.

Some of the techniques used by spyware/adware include,

Adding the spyware as a windows service.
Adding the spyware as a browser helper object which has unlimited access to system.
Adding the program into the startup program list.

One of the best tools to fix a spyware/adware infection quickly is the HijackThis tool. This is a lightweight freeware software which is only 220KB! Using this you can inspect the programs that are running as a service, the programs that are attached as a browser helper object etc. But be careful that you don’t delete the required programs!