@Aha2Y This isn't outdated. I use this script every day. Just tested again with a clean paste into mIRC 7.17 on Win7 x86 & XP Service Pack 2, mIRC 6.35 on Win7 x86, and mIRC 7.19 on OpenSuse 11.3 w/wine.
Many things can cause the 401 error. 1st check your mIRC settings. Tools->Options->Other->Lock: Make sure "decode" and "Run" are NOT disabled.
Another issue that could cause this is an invalid timestamp. This script uses $gmt but it is only correct if your system clock is set correctly. Twitters api is picky about this , so it is better if you change your time settings to automatically update with a time server.
If none of this helps you, please post back here and include your Operating system type. Please specify 32 or 64 bit.