My guess would be that the chipset and all other appropiate Drivers were not installed after upgrading to Win7...
Agreed.
Unfortunately Dell does not appear to provide Vista or Windows 7 Drivers for that model, since it's designed for Windows XP; Additionally, Dell's own information states that the model is not compatible with either Vista or windows 7.
The
driver page for the model does not allow the selection of Windows Vista or 7.
This explains the slow-down; a PC that just barely edges out the requirements for 7 that also is using default drivers is liable to have the "slowness" that is being experienced.