Winforms - OpenFileDialog Latency On Win7

I have a winform, VBExpress 2008 with a single button and this code behind it to illustrate the problem:

CODE:

When I select a file (in my case small PDF's) I am getting a 3 to 6 second delay after selecting the file until it goes on to the next statement, in this case a MsgBox.

If I repeat selecting that same file a second time, I don't get the delay. But, if I exit the program and re-start it, then I do get the delay again (one time). After the first one hangs it looks like it hangs about ever 7 to 9 tries and the window says "not responding" for a few seconds. I don't have this problem on Vista or XP.

If I select the same file over and over, it might hang the 1st, 2nd and 12th time. Close program try same again, this might be the 1st, 3rd and 9th time. Never the same - as if it is running out of resources and clearing a cache.

I've watched the task manager during the delay and don't see anything unusual coming up. I have a new computer with everything operating properly. I thought maybe it was my Norton Internet Security -- but I turned that off and it made no difference.

How I might trace the source of the delay to the bottom of it? I've tried everything I know to isolate the problem and am running out of ideas. I don't see any other posts about this particular thing.