Try cleaning out your windows temp folder.
If that didn't help:
Does it happen on the first workbook opened or all of them in that session. If
it's the first, maybe it's not the actual workbook that you're opening that's
slowing things down.
If that doesn't help, try starting excel in safe mode:
close excel
Windows Start button
run
excel /safe
Then File|open to test it out.
If that helped, then maybe it's your *.xlb file (where excel stores your
customized toolbar changes) that's causing trouble.
Use Windows Start button|Find, to find it (them?) and rename it to *.xlbOLD.
Then retest. If it helped, then dump that *.xlbOLD and recreate the custom
changes once again.
If it wasn't the *.xlb file that slowed it down (and safe mode worked quickly),
then it might be an addin that you're loading when excel starts.
Turn off everything in Tools|Addins and move all the files out of XLStart (but
make a note of what the names were and where you moved them so you can restore
them if this didn't help).
Test it again.
Chip Pearson has some notes about diagnosing start up problems at:
http://www.cpearson.com/excel/StartupErrors.htm
If that didn't help, maybe you can find something at these two sites
(Charles Williams and David McRitchie):
http://www.decisionmodels.com
http://www.mvps.org/dmcritchie/excel/slowresp.htm