Even though it's fixed, the following might be good for future reference:
Open regedit by going to Start->Run, typing in 'regedit' (without the quotes), and hitting Enter.
Now, navigate to the following folder. A visual representation:
In the right pane, there should be an entry for plugindir2_5. Make sure it reads C:\Program Files\AviSynth 2.5\plugins - if it doesn't, or it doesn't exist, then edit it to read such or create a new entry so that it does (right-click, New->String Value; name the string plugindir2_5, and edit it to match the right plugins directory).