fsx add-on plane problem! im not a newbie who doesnt know how to add planes btw!

i just switched to Windows 7 from windows vista and windows xp... actually my fsx worked fine with lots of third-party planes freewares, paid ones it doesnt really matter.... but now (i also switched to USAVA recently thats why i need to install the USAVA fleet.)... i need to add just two planes now... Dash-8 and S340... I tried with both automatic installer and manual install. neither one worked... when i install Dash-8 it works fine... but when i install S340 they both disappear from my aircraft list. i deleted Dash-8 and installed only S340 it worked fine too. but when i installed Dash-8 they both disappear again!! i thought it might be a problem about USAVA fleet then i tried 3 of my old aircrafts... they didnt work either. i mean they work individually but i cant have 2 different third-party planes at the same time... please help me!!! ugh i hate windows.... wish i had a mac :D