I also had the same problem as those above. [XP-Home, SP3, 3G Ram and over 100G free space on C: drive (less than 2 years old)]
I did a test with Avast's file system shield turned off for ten minutes and all my applications started up normally again (less than a second to open photo viewers, word processors and media players). Once I turned the shield back on my application startup times all slowed way down (anywhere from 8-15 seconds to get a splash screen!).
So, obviously, this is not my system or my hard drive that's causing this slowdown!
After a lot of clicking and testing, I found that if I disabled Reputation Services, it fixed the problem!
I'm not totally sure what protection I'm lacking by having that disabled (I don't use Hardened Mode).
But, at this point, it was either run it that way or find a new AV, because the apps were taking so long to open that I though I had failed to click and would click again - often resulting in two or three instances of the same app open!
Hope this helps others as well!