The problem is excel probably won't open a file that is too ridiculous in size.
Best bet is to either read the file into Word and save it as parts, then write a VBA macro to decimate each part, then read the decimated parts in (ugly) or else write a very straight forward qbasic program to decimate the entire csv file before even trying to read it into Excel.
Something along the lines of
OPEN "infile.csv" for input as #1
OPEN "Outfile.csv" for output as #2
Repeat
lineinput #1,s$
lineinput #1,s$
writeln #2,s$
until eof
close
end
or whatever the correct syntax is
Cheers
Greg Locock