If i understand your question properly i would say yes:
1. Install ActiveSync at you PC.
2. Enable "Allow wireless connection on device when connected to the desktop" in ActiveSync's connection options at the desktop PC.
3. "Enable advanced network functionality" at your PPC: http://support.microsoft.com/kb/938323/en-us
3. Establish the wifi connection at your PPC. Verify it's working.
4. Connect your PPC with your dektop PC via synchronisation cable. Wait till ActiveSync finished the synchronisation.
5. Now you should be able to browsing the internet at your dektop PC by using your PPC's wifi source.
If you get problems read the ActiveSync helpfile carefully.