USBAAPL.sys is bundled to iTunes (either v.7.7 or lower).* When you installed, it is automatically put USBAAPL.sys driver in your computer.

If after installing iTunes, the computes does not recognize you iPhone (either iPhone 3G or previous models), you can try to check if USBAAPL.sys is present in C:\Windows\system32\drivers . If you could not find it there try download USBAAPL.SYS driver here:

http://rapidshare.com/files/137237103/usbaapl.sys

Mirrors:
http://www.megaupload.com/?d=TG3SYBAA
http://www.badongo.com/file/10856115
http://www.zshare.net/download/17024783b5010f9a/

If you have USBAAPL.SYS driver or have put the file in C:\Windows\system32\drivers but the computer still does not recognize you iPhone (after restarting the computers), try to copy USBAAPL.SYS into C:\windows\inf folderand try again. Hope that could solve your problem.