Old DOS Sys Admin

I have a 6 month old Dell XPS that shipped with Vista Premium. It is a dual
processor, with 4 gigs of RAM and a 500 GB. When I first boot the computer
is seems to be sluggish and if I try to use a program it will freeze. This
happens in IE 7, video games and MS Office programs.

What I have noticed is that if I let the machine run for about 5 minutes it
seems fine. I have started the Task Manager and switched to the Performance
Tab - as the "Cached" number gets higher the system becomes more stable -
e.g. if I start a program as the "Cached" number is below 700 then I will get
a freeze - but when "Cached" is above 1,500 then it runs smoothly.

I feel like I am running an old 386! Turn it on and go make coffee!

Any suggestions?