i have this problem, i just can't sinc my Bold 9700 with my HP notebook pc running windows 7.
i installed the disc wich came with my BB, desktop 5.0.1.26, when i try to sync i get the same message, whether usb or bluetooth, "unknown error ocurred".
then i downloaded from blackberry.com current DS maneger version 5.0.1.28 and installed it but STILL GET THE SAME ERROR.
The latest version of DM (5.0.1.28) is fully compatible with Windows 7.
A couple of quick ideas:
1. try starting DM without the BB device connected via USB
2. Try changing USb cable..
3. Uninstall DM and manually delete folder inside Program Files / Common Files / Research in Motion, Use a registry cleaner, such as CCleaner, reboot and try reinstalling DM.
DM will NOt synch via BT until you configure it when first running. Try first to obtain synch via USB, then configure the BT synch.