Try downloading
MBAM (free version) install it, update it, run a quick scan. Have it remove anything found. If it prompts for a reboot to complete removal, do so promptly. Post the details of anything found.
There are other tools that can be used, depending on the problem, such as
LSPFix; you might want to flush the DNS cache. Easy way to do this is to right click the connections icon in the system tray, select 'repair'. The flush takes place as part of the repair, unless something doesn't want it to.
[edit] I guess you use the Windows firewall? That shouldn't be the source of the problem, if so.