or you can just re-download it again, if u don't want just search your computer for extension of ipa, those are your apps

cant remember which folder but it is in your itunes folder

if you sync back the iphone to your new pc, you will get the app back in itunes

anything that you bought can be redownload free, just check your appstore app, in the updates section you can view your purchases (4.3.3 firmware)