I want to create

Antivirus Software in vb.net
Though it not against the term of service, discussing the mechanism behind virus is. In it this second part that Anti-virus Software Companies understand well.
For more opinions read the Antivirus Creator Thread

an OS (Operating System) using vb.net
Definition of Operating System
It is theoretically possible but itsn't an easy thing to do.
An example of a C# derived OS see Singularity
Note: That it use some lower level languages in its creation

a KeyLogger using vb.net
Yes it is possible to do this. Though it can be used with good intentions it can also be used maliciously to obtain login credentials like user name and password.
The ethos of the site is more towards create good helpful software and it also against the Terms Of Service of the site.

my own Programming Language using vb.net
It is entirely possible to create a programming language in vb.net, or one that compiles to .net.
Simple Example: PenScript (Based on Logo)
.net Language
Based on DLR
Book: Build Your Own .NET Language and Compiler