I recently had to help someone who lost their internet connection after removing a virus from their system. It was a desktop Compaq Presario running Windows XP Home SP3.
The infected file was netbt.sys which is an important part needed for your internet connection. The computer claimed it was connected, but the troubleshooters built into Windows XP could not find any problem but there was one because there was no access to the internet.
Using the ipconfig /all showed that the computer was connecting to it's DNS servers but the IP address was listed as 0.0.0.0. I tried the usual resetting and repairing of the connection, resetting the winsock, flushing DNS, etc., but nothing seemed to work.
Searching through the Error Event logs in the Control Panel it showed The DHCP Client Service depends on the following nonexistent service NetBT error: event ID 7003.
Some research online and I replaced the file netbt.sys to the Windows\System32\drivers folder but it didn't make any difference. Finally I decided a System restore. It worked, problem solved ... or was it?
It did work for a couple of days until the same problem developed again. Originally I replaced netbt.sys with another copy found on the hard drive. This time I tried to replace it from my Windows install CD's, it it kept complaining that it was not the right version of Windows. According to the system info from Windows, it was Windows XP Home SP3 my copy was Windows XP Home SP3. Now it all made since ... IT WAS WINDOWS! :-)
I replaced the netbt.sys, didn't work. I was going to use my CD to do a System Repair Install but it was already complaining about my install not being proper. Same thing with SFC - System File Checker. I needed to get rid of the old corrupted files and replace them with new ones and after scanning over the system setup I decided it might work if I uninstalled the updates Service Pack 3 - SP3.
So I did. I got some warnings about programs that may not work properly, that was OK. I was going to reinstall it again after and hoped everything worked out for the best. Once uninstalled the internet started working automatically, but they where right about things not working properly. The browsers IE and Chrome kept crashing and the anti-virus wouldn't work.
So I reinstalled the SP3 updates and everything seemed to be working perfectly again. It's been a couple weeks now and I'm taking the silence as a good sign.
Just a note when I tried to renew the IP address, when it was still broken I would get the error message The RPC server is unavailable. Also when I uninstalled SP3 the system reverted back to SP1, maybe the reason my CD's where useless in this case.