Connection drop out each session

My internet connection drops out each time the Insprion 1720 is put into sleep mode or even re booted.  There is another Inspiron computer on site with similar specs that does not have a problem at all.

The 'problem' computer is running Vista version 6.00 non SP1 . (SP1 does not come up as an option in the update list).

If I reboot the modem and wireless router, it does repair the problem. It has only started recently and it seems as if the problem is either software or setup related.

Performing a diagnosis does not repair it.

THe modem and router are different makes.

Sometimes I get the 'blue screen', but not always. Microsoft reports that this is repaired by installing SP1. I will consult M'soft on that as a separate issue.

Does anyone have any clues?