A lot of issues have popped up concerning the Dell laptop Bluetooth module on Windows 7. Some laptops, such as the D630, seem to work out of the box. Others, such as the D620, do not. Some people blame Dell for not rolling out new drivers, other blame Microsoft for not pushing fresh drivers through Windows Update.
Many fix-it ideas are floating out there, but a simple approach has worked for me:
Go to the Dell site and install the Windows XP 64-bit drivers for your particular model - you ARE running 64-bit, aren't you? This should get things functional. But you may still notice timeout and sync issues.
To solve this, go to Device Manager. Open the Properties for the Dell Wireless 3x0 Bluetooth Module. On the Power Management tab, uncheck "Allow the computer to turn off this device to save power"
That should get you production ready until Dell (or Microsoft) rolls out fresh drivers for these modules.