Simple project

There are two files: A .csv file contains: "UserID","Email","FirstName","LastName","Address1","Address2","City","State","ZipCode","Country","HomePhone" "andykin456","andy123_48043[@][login to view URL]","William","A White","57 EASTMAN STREET","","Mount Clemens","MI","48043","United States","xxxxxxxxxxxx" "andykireitov","andrewkireitov[@][login to view URL]","Andrew","Kireitov","6771 1/2 Yucca Str. App #7","","Hollywood","CA","90028","United States","xxxxxxxxxxxx" "andylebatt","riflemaster[@][login to view URL]","Martin","Collins","138 Sandy Lane","","Rochdale","Lancashire","OL11 5DR","United Kingdom","xxxxxxxxxxxx" "andykiss2561","[login to view URL][@][login to view URL]","Andrew","Smith","15 Laurel Close, Adeyfield","","Hemel Hempstead","Hertfordshire","HP2 5ET","United Kingdom","xxxxxxxxxxxx" "andykissfan","andyharris[@][login to view URL]","Andy","Harris","100 Kenn Road","","Clevedon","Avon","BS216EX","United Kingdom","xxxxxxxxxxxx" "andym3851","papabear17[@][login to view URL]","Andy","F Maynard","PO Box 995","","Grundy","VA","24614","United States","xxxxxxxxxxxx" "andy-kitt","andyp11gt[@][login to view URL]","Andy","Graham","21 Kinsale Avenue","Hellesdon","Norwich","Norfolk","NR6 5SG","United Kingdom","xxxxxxxxxxxx" And a .txt file contains: boehmerwilly deejaymotz roxi314 viper20m heikeline1982 merzbar99 biking-tiger reapoplex mufti0206 grossermagier hu650 maxi700 hojobecker hans4262 wulfram2005 ebannerto stine_1968 plietenfischer28 The txt file contains only users ("UserID") one per line. I need a script that can check if the UserID's from the txt file already exists in the .csv file. The UserID's that doesn't exists in the .csv file should be saved in a OUT Put .txt file, one per line. The script can me made in any write language you preffer but it should work on windows platform. The code must be very fast. The both .csv file and .txt file are very - very big ! So there need to be a clean code that will not block my pc. Have questions? PM here.