It might be too late now, but there ARE a couple pieces of software that will do exactly that. The easiest one to use being USBLogView
Other options, not so friendly, are the Windows USB Storage Parser or Microsoft's USBView (UVCView on Win7), that ships with the Windows Driver Kit (WDK).
If you really want to get your hands dirty, open RegEdit and look for the following entries:
Description: List of Installed USB devices, both connected and
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB Why you care: It
can be useful to know what USB devices have be connected to a box, and
even the vendor and serial number of the device in some cases. Think
someone copied the data to a thumbdrive? This may help you trace down
what thumbdrive. Think how useful it can be to help tie something a
user physical possesses to a box.
Description: List of installed USB storage devices Location:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USBSTOR Why you care:
Much like the installed USB devices entry, but just for USB storage.
Think someone copied the data to a thumbdrive? This may help you trace
down what thumbdrive. CleanAfterMe scrubs
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB but not USBSTOR
when I tested last.