It sound as though your hard disk is failing because of overheating. When the laptop is cool, it works OK and loads eg Word, but into memory. Hence you can continue to run Word. after a while, the hard drive becomes too hot and fails, so you cannot save, open a different file in Word or change programs.
I am presuming that Slax is running from the CD in memory and not being installed.
You could try one of two things:
1. if you think I may be right (and test it a bit yourself), send it back to Evesham again, telling them your suspicions and that you expect them to test specifically for this
2. have it tested by an independent specialist, explaining your suspicions and if he confirms the problem, go back to Evesham and demand your money back or a repair on the basis that the fault was inherent. Note that they may not give you all your money back because you have had some use out of the machine.
I hope that you are able to solve this.