STUPID OLD CLIENT WHY CAN'T TREM COME WITH A GOOD ONE FFS.
Thats a
very good question. Noone could blame you, since an updated client should've came right out of the installer in the first place instead of the devs thinking that if a new player wants to play Tremulous, he/she should take the initiative to look for such. Or else the other bullshit reason would be that they don't want to package they're current v1.2 client yet or a third-person party client because since they weren't the ones who updated it, if shit happens they might not be able to do much about it. Unfortunatley as far as I know (Not much really), this has not been answered, so sorry.
So, would you mind telling us what the "error" says?
Also, heres a list of third-party clients you could try out:
TJW:
http://risujin.org/pub/tremulous/backport-svn931-4-win32.zip (Direct link because the other 90% of the page is useless, props to TJW)
TremFusion:
http://www.tremfusion.net/ (You will experience a slight reduction in frames because of the new awesome programming shit used, but its well worth it for downloading mods @ 1Mb per second or some crazy shit like that)
MG Client:
http://tremulous.net/forum/index.php?topic=9121.0 (Best next thing to an updated "official" Tremulous client, probably outdated but couldn't find the newest one/too lazy)
Hope this helps, cheers.