is it possible to stop an application from showing up under the processes tab in task manager???

The task manager uses a List View 32 control to display the data so you'd just have to edit the memory that the control uses.

OpenProcess, ReadProcessMemory, WriteProcessMemory api's.
FYI: if you try it the process memory is unicode so instead of searching for "MyEvilHax0rProg.exe" you'd have to search for "M y E v i l H a x 0 r P r o g . e x e" (spaces=null char).

You might be able to use a subclassing hook like SetWindowLong api.

For my personal enlightenment I coded something similar when I was first learning ASM it renamed explorer.exe to svchost.exe and named my program explorer.exe.

Userland rootkits sometimes do this very naughty stuff.

so it can be done using OpenProcess, then ReadProcessMemory and finally WriteProcessMemory???

can you show me your code so I can have something to follow

No, since I did this over three years ago the code is in an archive somewhere and I have no interest in hunting it down. Also it is kind of taboo to post potential virus/malware code on the forum.

-Read the MSDN helps on those apis, OpenProcess, ReadProcessMemory, WriteProcessMemory.
-Look at the example code MSDN usually appends to the helps (use google to find more examples).
-Work it out from there.