Normally caused by pgpfsshl.dll (on PGP 10.1.1 & 10.2  Not sure if fixed in later releases)

If you use Process Explorer and monitor it’s threads at the time of the issue, you’ll probably see pgpfsshl.dll in there running at 50% or more.

This feature of PGP scans a directory for files that are encrypted so it can then display an icon showing the file is encrypted.

Causes this ANNOYING issue.  So can safely be disabled by running

regsvr32 /u pgpfsshl.dll
Reboot might be required before notice a difference.