You have a malware infection. Reboot into Safe Mode with Networking, keep tapping F8 as you boot up, then use up arrow key to highlight Safe Mode with Networking and let it load.
If Internet Explorer won't stay open clean your Hosts file:
Double click hosts File and select Notepad to open it.
Scroll down to the line BELOW
127.0.0.1 localhost and delete all entries, then click File and select Save As and save it in C:\ Windows\system32\drivers\etc, do NOT change the filename.
Then download the following free programs:
Norman Malware Cleaner - No install required, will also run from usb flash drive or CD, simply download, double click to open, Accept Agreement then Run:
http://download.norman.no/public/Norman_Malware_Cleaner.exe
Kills running processes that are infected
Removes infections from disk (including ActiveX components and browser helper objects)
Reveals and removes rootkits
Restores correct registry values
Removes references created by malware in hosts file which redirect to other sites and block downloads
Removes windows firewall rules for malicious programs
Alternative download site:
http://www.download.com/Norman-Malware-Cleaner/3000-2144_4-168467.html?tag=mncol
Malwarebytes Anti-Malware:
http://www.malwarebytes.org/mbam.php
Select Full Scan.
Remove all infections that it finds after scan.
SUPERAntiSpyware Free Edition:
http://www.superantispyware.com/downloadfile.html?productid=SUPERANTISPYWAREFREE
Select Scan your Computer, select your drive and select Perform Complete Scan and remove infections.