We need a closer look at what's happening.
Please download http://www.merijn.org/files/hijackthis.zip
Copy it into its own folder, doubleclick HijackThis.exe, and hit "Scan and save log".
When the scan is finished, the log will open in Notepad. Do Ctrl-A to Select All, and copy its contents here. Most of what it lists will be harmless or even essential, don't fix anything yet.