Vista runs a service called ‘superfetch’ after the desktop loads which generates lots of disk activity. This can slow down the system for a period (on mine it’s around 10 minutes). However it still only normally takes seconds to load most programs and speeds up once the disk activity has settled down.
It’s possible that Dell has installed loads of ‘crapware’; trial programs or utilities that start when Vista starts up. You should be able to disable these using msconfig or uninstall them completely.
Why it should take 2 minutes to load firefox though I don’t know