What kind of Antivirus and Anti-Spyware do you use? Do you use any firewall other than Windows Firewall? Which service pack you are running Windows SP1/2/3? Where from you downloaded IE7. There are website which distribute altered IE7. Download it from http://www.filehippo.com/download_internet_explorer/3282/
Your computer seems to be infected with malware. If you have no antivirus or antispyware installed then you can try this
1. Boot in safe mode. Press F8 several times during computer startup, select safe mode with networking.
2. Download SuperAntispyware from http://www.filehippo.com/download_superantispyware/
. Install and update it.
3. Run the scan and clean if it detects any malware.
4. Download avast! antivirus from www.filehippo.com and repeat same process.
Have a nice day.