Can't connect to machine from Performance monitor

I am try to look at a couple of machines activity at a certain time of the
day. I already use a Protocol Analyser to watch the network, but I want to
be able to monitor a few machines activities to see if they are busy.

I was going to the Performance monitor and watch the Processor(% Processor
time) to see if it is spiking. The problem is that there is one machine I
can not seem to connect to, but I can connect from my Windows Explorer.
Other machines I can connect to fine. There are some I can't, which I don't
have access to - but this one I do and I can see most of the folders (there
are some that don't show up - including the Winnt folder). Is this what
would cause me to not connect?

What does it mean it can't connect? Does PM mean I can't connect to certain
folders?

Also, is this the best counter to use to see usage? We are looking to see
if we are having a DOS attack.

Thanks,

Tom.