The two closest programs to Adium on Windows are probably Pidgin (Pidgin is the major open source project on which Adium is based -- that is the chat protocol underpinnings in Adium come from Pidgin) and Trillian. I personally like Trillian, although people seem to love it or hate it.
Unless you need anything specific (Photoshop, Office etc...) then Windows as most of it covered for you.
CD burning is taken care of by WMP,
Like you said VLC will be spot on for video playback,
and if you want an IM, Windows Live Messenger will do the job.
As for DVD's, W7 does have DVD burning software built in but I've never used it so I can't say what files it will support.
I've never used it as I just don't use DVD's these days. It's all streaming and USB for me.
I still do the odd CD for the car but I have a HDD in the car with all my music on so it's only once in a blue moon I copy one. Optical storage is just a bit old school these days.
If you find that the DVD software doesn't cover what you need try Nero.
You can pick it up for £2 or so if you get an OEM copy.
As for security, I personally run Kaspersky, but that's just me. I like the flexibility of network management of applications.
The MS Security Essentials package is very good, and free things that are good are always nice.
In short, what you've got already should do you just fine