HELP!!!! MSN sending emails automatically

Hey to whoever can help me....

I logged onto MSN Messenger a few days ago and discovered that my account has been sending emails to all my contacts without me doing anything.

The same day all my contacts were deleted from Messenger and when i reentered them they all came up as blocked.... I proceeded to go into options, then in the privacy tab where I unblocked my contacts and discovered that one of my blocked contacts had been changed to allow????? ...I did not do this.

I am running Avira Personal as my anti virus and when I do a scan, 3 warnings (that I have no idea how to delete, or really what I am suppose to do with) come up but nothing else.

My PC is now running really slow, it takes an eternity to start up, is browsing really slowly, occasionally freezing all together and I have to go into task manager to end task....

I am a complete novice when it comes to viruses, and am pretty new to computers..... So if anyone can help me (in very basic terms :) ) I would really appreciate it......