Rootkit is one of worst kinds of malware you can get. It hides itself somewhere in your computer system and does many damaging things such as stealing all your existing and future data. It even opens a back door that allows the hacker to send data back to another computer discreetly without your knowledge. Written elusively, a lot of common anti virus programs can't even detect their existence. Always run a strong anti virus program in your computer background for safeguard from now on.
If you notice your internet connection speed is erratic, run the following rootkit removalists and try:
http://www.f-secure.com/en_EMEA/products/technologies/blacklight
http://www.sophos.com/products/free-tools/sophos-anti-rootkit.html
http://www.trendmicro.com/download/rbuster.asp
Some of these are free and they are all reputable tools. Good luck.