For ex: i have an aaa.exe running in windows startup and visible in windows startup, What i want is to make aaa.exe invisible or make aaa.exe replaced by null characters .

Yes it is possible. I must however warn you that you will be writing into your systems registry to make it possible. Make a backup of your drive BEFORE attempting any changes to registry. We at Daniweb or anywhere else do not take responsibilty for any system crashes......
See the attached txt document for the code that will put you on the right track.

Good luck.