It is possible that it may just be the web browser that has been hijacked. In which case, you may be able to get around this by downloading another free browser and installing it on the machine. You may have to use another clean machine to download the installation files for the browser onto a removable media e.g. CD or USB and then transfer the files over to the affected machine and then run them on the machine.
e.g. You may be using Internet Explorer to browse and may have to download alternative free browsers like Mozilla Firefox or Google Chrome
If this works then you have temporarily bypassed the browsing issue but I would still recommend that you refrain from browsing on this machine, especially to sites which require a Username and Password as you dont know what may be on the machine.
Can you please follow these instructions initially but also let me know what you use as your primary browser and which version e.g. Internet Explorer version 7.0 or 8.0 etc
If you suspect you're infected and have trouble finding what is causing the infection, I'd suggest giving this handy tool a try.
"McAfee GetSusp is intended for users who suspect undetected malware on their system. By using a combination of clever heuristics and querying McAfee's online database of known clean files to gather suspicious files, GetSusp eliminates the user's need for deep technical knowledge of computer systems to isolate undetected malware. McAfee GetSusp is recommended as a tool of first choice when analyzing a suspect machine."
Get it from here:
Once GetSusp identifies and collects the suspect files, post the logs here and we community members can help.
Technical Product Manager, McAfee Labs