Marshur,

Using the "search" option in this forum would give you a number of threads about this subject. Click on the link below to read one such thread with a number of solutions. You'll need to read all of the posts to get the information you need:

Autoexec.nt Repair Thread

If it continues to happen, then I suggest running some tests for spyware and do a system clean up. Here are some links:

Free spyware removal tools to install:

Ad-Aware">http://www.lavasoftusa.com/support/download/]Ad-Aware