The only things I could suggest are to clean up your background services (with msconfig) and startup applications (with CCleaner), and other general system maintenance like flushing temp caches and cleaning the registry (both with CCleaner). The latter just being good overall advice; the background and startup stuff is more relevant, as it'd reduce the CPU/memory usage enough that you could get some better performance.
Another general suggestion would be to upgrade to AviSynth+
*, in which case you might be able to get a performance boost by using multithreading and/or 64-bit (64-bit doesn't have as many plugins available for it, but if you're not using any except FFMS2 - which has 64-bit builds available - then there's little harm in at least trying it; if using the 64-bit version, you also need to use 64-bit VirtualDub). Not exactly sure how multithreading in AviSynth might interact with VDub, but again, it won't hurt to try.
*the stable release on the official website (r1576) doesn't have multithreading, as it was the last build before MT was introduced; for the MT version (r1689 or higher), see this post on Doom9 (and the one after it) for the notes on how to use it:http://forum.doom9.org/showthread.php?p ... ost1666364
Due to the way the new cache is designed, the MT build should provide lower memory usage even if you use it in single-threaded mode.