Because of changed made to registry entries and protected filesystems, local Administrator rights are normally required in order to install PyMOL under Windows.  Did the IT department recently take those away perhaps?
If not, then maybe the tips posted on PyMOLWiki can help:
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From: pymol-users-bounces@lists.sourceforge.net [mailto:pymol-users-bounces@lists.sourceforge.net] On Behalf Of David A. Horita
Sent: Wednesday, February 13, 2008 10:52 AM
To: pymol-users@lists.sourceforge.net
Subject: [PyMOL] pymol nstall problems

I've been unable to install Pymol recently on an XP Pro-SP2 system.  I'll start setup.exe, and ntvdm.exe starts running but nothing progresses from there (no error messages and installshield doesn't start).  I'm not sure if it's a software conflict with something else running, but it's likely (and the PC is managed/regulated by the IS department, so I have limited ability to muck around with it, and they like to push patches in the middle of the night).  At one time I successfully installed 0.99rc6, but now that install also fails, so something has changed.
I've seen other posts where people have had problems with the version of installshield on xp; I've cleaned out my temp directory, and tried running setup in various compatibility modes and with its own memory space, and in safe mode.
Has anyone else seen/fixed this problem?
David A. Horita, Ph.D.
Department of Biochemistry
Wake Forest University School of Medicine
Winston-Salem, NC 27157-1016
Tel: 336 713-4194
Fax: 336 716-7671
email:  dhorita@wfubmc.edu
web:  http://www1.wfubmc.edu/biochem/faculty/Horita.htm