See MS KB 263455 - "Antivirus Tools Cannot Clean Infected Files in the
_Restore Folder" (http://support.microsoft.com?kbid=263455).

There is no need to be concerned about any virus or trojan in the _RESTORE
archive as they are harmless there and can only cause problems if you
later choose to restore to a checkpoint created AFTER infection and BEFORE
you cleaned your system. Any worms, trojans and viruses in the _restore
archive will automatically be discarded in time as newer data is archived
and older files discarded.  The problem with disabling system restore is
that it flushes the _restore archive and whilst that removes any virus
remnants it also removes any good usable checkpoints you might have.
Therefore only clear the archive if you are happy with the current state
of your system and are having no problems that might be solved by using
system restore.

Moving on to how to clear the archive.  There are two approaches to
resolving your problem:
Firstly try reducing the space allocated to the System Restore archive as
this could flush out these unwanted files.  Do this using the slider found
at System | Performance | File System | Hard Disk and reduce the allocated
space until you flush out the unwanted files.

If that fails, reset System Restore:
System | Performance | File System | Troubleshooting and check "Disable
System Restore", Apply and IMMEDIATELY reboot.  This will flush you
restore folder and erase all checkpoints, then,
System | Performance | File System | Troubleshooting and uncheck "Disable
System Restore", Apply and again IMMEDIATELY reboot.  This should now
automatically create a new checkpoint immediately following the restart.
Finally adjust the space allocated to the restore folder,
System | Performance | File System | Hard Disk and adjust the restore
slider to your preferred setting.  A  figure of 200MB is normally more
than adequate for day to day use allowing perhaps a week of checkpoints to
be available although increasing this to perhaps 400-500MB for a few days
during periods of large installs such Microsoft Office is advisable.
--
Mike Maltby
mcmaltby@hotmail.com