First make sure it isn't a virus. Download TDSSKiller and RKill from our
Freeware DB. If you do not have an anti-virus then download, install, and allow to update Malwarebytes Anti-Malware (MBAM). Then reboot your system into
safe mode without networking (Reboot and, when it says "Starting Windows," Press F8; If you get a message asking continue in safe mode or perform a system restore, choose continue in safe mode). Now run TDSSKiller and after that run RKill and after that run a full virus scan. The first two will take maybe 5 minutes each but a full virus scan can take anywhere from 30 minutes to several hours depending on your hard drive size and number of files.
TDSSKiller is designed specifically to look for rootkits. RKill is designed to stop the stealthing programs that hide/protect/regenerate some nasty viruses. Running these first will help ensure your A/V has the absolute best chance of cleaning up infected files.
Assuming it is not a virus then go to
Windows Update. Click on
View Update History. If it isn't already, sort the list by Sort the Look by Date Installed and make note of any in the last 10 days, most importantly, Network-related Drivers (NIC cards, etc)
Then click on
Installed Updates and uninstall any recent drivers (if they are listed). If you saw one on the previous screen but see nothing on this one then you will need to go to control panel > Device Manager and locate that item. Open properties for it and choose "rollback driver".
.... Those are my only ideas other than a fresh W7 reinstall or restore from an image backup if you have one.