Quite often, one of the applications I use freezes (“does not respond”) for a while, in extreme cases for few minutes. This happens especially when when switching apps. During this time, the HDD light flashes constantly and perfmon show that HDD is used 100% of the time (OTOH, CPU isn't) and that pagefile is being read (which is to be expected when switching apps), but at a very slow rate. When I sort the disk table in perfmon by read or write, the file read and wrote the most is the pagefile, but it's still quite low rate (I don't remember the numbers).

How can I diagnose what's causing this?

I use Windows Vista, and the computer is quite ordinary two years old laptop.