Check your IE browser addons for anything that you do not recognize or are not certain
as to what they are. Either do a search for the addon or ask here.
Check to see if you have or have not disabled AutoComplete in IE. How to disable or enable Internet Explorer's AutoComplete. Disable Internet Browser Add-ons/Plug-ins - Internet Explorer | Yahoo! Customer Care Troubleshooting Help
Disable Internet Explorer 8.0 add-ons:
1. Click "Tools" and select "Internet Options".
2. Click the "Advanced" tab on the right.
3. Under "Browsing", remove the "Enable third-party browser extensions" check box.
4. Click "OK" and close out of all instances of Internet Explorer.
5. Relaunch Internet Explorer.
Check the extensions and plugins in Chrome. Manage your extensions - Google Chrome Help
Always a possibility that malware is involved. Though I do not know of any that do what you describe.
Have you scanned for malware? If so, with what programs? How To Disable Individual Plug-ins in Google Chrome