Not a perfectly secure solution, but from the FAQ:
http://securityadmin.info/faq.asp#runas
[only the Microsoft articles on creating a Runas icon apply to your
question.]
There is a free utility out there that will let you schedule the Win2K
defragmenter, or Diskeeper Pro [or something like it] will let you schedule,
monitor and manage this remotely. Note that there are other advantages to
running the paid version of Diskeeper as compared to the free one that comes
with Win2k, that you can read about on the diskeeper web site.
These are IMHO two of the most typical solutions to this problem.
I believe scheduling defrag is better than making users run their own
defrag, because that causes unnecessary extra manual effort that becomes
expensive across a large organization, and also some machines will never be
defragmented. On newer machines that have lots of free space [e.g. more
than 30% to 50%], running defrag frequently is probably not as necessary or
as helpful to performance as it had been in years past.