I noticed that I wasnt alone with my playblast problems so I figured I would post my solution. There is a regristry problem in Maya 6. that will not allow you to R-clik and render from the icon in your scene folder. With help from The IT guy at my school I have fixed the regristry problem and as a result it seams to have fixed the playblast problem as well. Following is a copy of the email he sent me. You will have to make some changes to your regristry but it is actully pretty simple.
First Go to start/run then type in regedit. This should bring up your regristry and the rest is simple.

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HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MayaBinaryFile \shell\render\command

you would see a value like:

C:\Program Files\Alias\Maya6.0\bin\render.exe %1

you need to change that to

"C:\Program Files\Alias\Maya6.0\bin\render.exe" "%1"

you'll want to make the same change to
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MayaAsciiFile\ shell\render\command

to get ascii files to work.
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Don't forget to restart your computer. I am no techy but I think this should fix the problem. Good luck!