First and foremost I would do all the usual and clean the system up. If you need a firewall and other anti-malware tools you can hit up http://www.download.com
and get the following:
Comodo (a free yet decent firewall app)
Ad-Aware (Malware removal for the price of free)
Spybot (Allow not the greatest anymore, it's still free)
Bazooka (Another free malware removal app)
AVG Free (free anti-virus program)
I would install these, update them, and run full system scans with all.
Next, if it's just certain sites that don't come up I would clear the cache and cookies from the browser.
In Internet Explorer you click tools and go to Internet Options.
In Firefox you just click on tools and then clear private data.
I would do these first and foremost and then go from there.