Thanks for the replies.
As I was expecting, the programs I installed on my second drive work. They just seemed to create new registries on the spot. However, a few didn't work; like marcellis said, the programs that use Windows APIs will have to be reinstalled.
Another thing is (sorta off point, but it was mentioned) that importing users from XP to Win7 is a breeze with the Easy Transfer Tool. I only imported my main user, but it looks like everything is how I left it before the install. Back-up's still recommended.
Conclusion: Yes, you can run Steam, play CoD4 and BC2 while talking on vent, open Y! Msgr while listening to ur music in WMP, all after a "hybrid-clean" install of Win7 from XP. : D