For some web sites I visit, an error box occurs saying

A runtime error has occurred

Do you wish to debug?

Line: 0
Error: 'dMTkc' is null or not an object



How do I fix this?

Try running the latest version of ad-aware.

Error Message: Runtime Error 216

SYMPTOMS
When you start Windows, Internet Explorer, or a Windows-based program, you may receive the following error message:

Runtime Error 216 at Address
CAUSE
This issue can occur if your computer is infected with a SubSeven Trojan virus.

This sound like you? If so goto:
http://support.microsoft.com/?kbid=259279

Good luck

ADDENDUM:

There appears to be a technique for disabling such errors - though I cannot vouch for its integrity:

http://www.google.co.uk/search?q=cac...=UTF-8&strip=1