i just find two methods of doing so under windows mobile 5.
however, i don't have same phone with you, and the settings i've found is made for my country. hence i do not guarantee it will work. however, i'll translate one of them for you:
1. Turn off your ActiveSync in your computer
2. Execute Auto Config GPRS in your PDA based on network you're using.
3. Plug in your USB cable to the computer
4. in your PDA, start --> program --> Internet Sharing
5. PC Connection --> USB
6. Network Connection --> your own GPRS settings (i.e. Tmobile)
7. Then hit " connect" and your PDA will be automatically connected to the GPRS
8. When it's done. you could start using internet in your compuer
9. If your computer prompt you for modem driver, you could obtain the file from CD that come with your phone.
okay it's getting late here i need to go to sleep
let me tell you the keywords to find the proper method in google:
WM5 bluetooth internet sharing.
Good Luck.