How to write an application that opens windows with an RFID tag

I need to write an application that locks the computer that launches windows 7. Locking is done by clicking on the lock icon or due to inactivity.

What's especially important in my case is that the unlocking is done using one of the RFID tags .

I got an RFID reader that simulates key presses when a tag is presented.

Decision

I was thinking about implementing a screensaver that checks RFID against a table of legitimate users, but I'm not sure if this is a good approach as it cannot be bypassed by pressing CTRL-ALT-DELETE, ALT-Tab, ....

I'm not looking for high level security here, just a way to prevent easy access to your PC.

How do you approach this problem? Do you think splash screen idea is the way to go?

Update

From this answer I know that it is not possible to unlock a secure desktop (with a secure one, I mean locked with a Windows+ L) using an RFID tag. But perhaps I could prevent access by not using Windows Desktop Lock ...?