Magic Trackpad / Mouse very spongy on Windows 7 with Bootcamp Update Drivers
I have experienced this as well (from MS mice in O SX and Apple mice in Win7).  I have reported this issue -- I think Apple's "doesn't work well with Windows" bugs get tossed on the back burner and forgotten (until they start to cause a stink).

In the mean time, this is what I have observed:


Completely disconnecting hardware before restart helps (Unpair/disconnect/turn off everything)
Microsoft USB hardware needs to be physically disconnected during/before reboot to work well in OS X
Plan B -- Use VMware Fusion to run Windows.  How much is it really necessary to have fu only for serious hardware needs.  I do this just so that I can use my magic mouse and bluetooth keyboard without trouble.
Have a cheap MS USB wireless arc mouse handy (the top-heavy laser resembles behavior of the magic mouse)


Other thoughts:


I had this problem in Ubuntu also -- if my MS mouse was left connected during a reboot from Win to Ubuntu (and Ubuntu to Win), the mouse would act funny.  The problem was solved by disconnecting and reconnecting.