Attempted to read or write protected memory

[Guest]

anyone ever seen an exception similar to this:

Attempted to read or write protected memory. This is often an indication
that other memory is corrupt

I'm opening .tif images in ASP.NET 2.0 vb.net App:
IIS 6.0 Server 2003
separate application pool with single thread
storing session data in stateServer

basically a treeview list of .tif images and when the user clicks on a node
the .tif gets renamed to sesssio ID Plus original filename (to "check out"
the the image, aka "in use")...when they're done viewing it, rename to
original by removeing the session id prepended onto the original file name...