Macbook Pro barely working after connecting to Plusnet

[#1]

i think other peoples "related" issues are clouding your judgement here.
your macbook appears to have a hardware fault and that has no relation to you switching to plusnet in any way.
there is absolutely nothing the plusnet router could do to make your macbook fail.

it has nothing to do with IPv6 or anything else related to the router.

[#2]

IPv6 is a complete red herring for your current problems, except insofar as it caused you to fiddle with your Mac settings which might, possibly, have gone wrong and done some other damage.  Symptoms described by other people on the forums surely only pertain to those programs that rely on an internet connection.  So you can

• Blame the router - but we cannot see how it could be to blame except that it might have caused you to fiddle with the Mac settings.
• Blame yourself for fiddling with the Mac settings.
• Blame bad luck and coincidence.

[#3]

@jaki the IPv6 issue is only specific to Mac OSX Lion and above, all previous OSX versions will not suffer from the problem.

As to the MacBook itself, have you tried the usual startup key combinations? (safe mode, reset pram and such)
What's the specific version of MacBook? I assume it's an intel one if you were thinking of upgrading to Snow Leopard.
If you still have the OSX installation disc, try starting up from that using the 'c' key (or 'option') at power on. From there you can run DiskFirstAid to check the hard drive for errors (it won't show data corruption errors but will show file structure problems). You can also try to back up your data while running from the install disc.