laptop boot problem

[Batman7070]

I have a Dell Insperon 6400 laptop with a dual core processor, its about 6months old and recently its started playing up (about every 4th or 5th boot up) viz:

Press the "on" button - screen wakes up .
Cooling fan comes on at max power.
Boot speed is much slower than normal - its as if only one of the processor cores is working, Any use such as collecting emails, surfing or WP etc is very slow and jerky.
Fan (still working flat out) only stops when I shut down laptop. At next boot up Laptop works perfectly - fan seldom comes on and after its cooled the processor it switches itself off.

It does not seem to make any difference if the Laptop is working on its battery or its power adapter. I always ensure that the cooling slots are unobstructed.

Any ideas? solutions?