This particular condition would be regarded as unacceptable and unreasonable in a court of law.
As you state, by their very nature computers are more than likely to be upgraded or checked for problems during their lifetime.
The computer is your property and I would suggest that you contact your own local Trading Standards department (they are always most helpful) and ascertain the true facts; usually contact can be made through the local council's main phone number.
It could well be that the company involved has made this allegation about the warranty becoming void for its own reasons - perhaps the specification isn't exactly as stated, for instance, or the workmanship leaves something to be desired.
No reasonable or reputable computer systems builder would attempt to impose such an unwarranted restriction on its customers.