> in the flow control, number of connections, you can 
> type in any number, say 99 or 999.  HTTrack really 
> will open the number of connections.  But, very 
> unfortunately, if the number is high, say 99 or 150, 
> my system will crash with blue screen death.  Is 
this 
> a bug or that there is something wrong with my 
> settings?
No, this is a system limit, which HTTrack can not 
know, and therefore no control is made by the GUI.
Generally, you can not have more than 64 simultaneous 
connections, and therefore the GUI only show choices 
until 32.

But it is needless to say that more than 32 
simultaneous connections is quite dangerous, except 
for internal website stress tests (I know some 
webmasters who are testing their website strength 
using such settings), and generally 8 connections is a 
good value (less if you have a fast pipe or 
downloading large files)