It doesn't sound good, since 16-bit subsystem errors are pretty classic signs of a malware infestation. Of course it could also just be that you have a bad extension installed.
First I would go in and disable ALL of your extensions in Firefox, then restart and see if the message disappears. If yes, then keep enabling them one by one until the error message comes back. You have to restart Firefox each time you enable a new extension. Once you find out which extension is causing the problem, you can hit Google to see if others are having similar problems and if they've solved them.
If the problem persists even with all extensions disabled, then grab some malware removers.
Also, since you're running Windows 2000, do you have some third party firewall? ZoneAlarm or a router perhaps? If not, it's not such a good idea to have that system connected to the Internet. You might also want to consider upgrading to Firefox 3.0.1. Not so many new features, but a lot of under the hood type improvements. Lower memory usage, faster page loading, more accurate page rendering.
Whenever I open up FireFox for the first time, or click on a picture that then tries to open in a separate window, these errors pop up:
A black DOS box pops up with a flashing cursor in it. The blue header bar on the top reads:
Then an error message pops up and reads:
"16 bit MS-DOS Subsystem" (...In the top blue bar)
The NTVDM CPU has encountered an illegal instruction.
CS:0dd3 IP:046e OP:63 74 6f 72 2e Chose 'Close' to terminate the application.
And of course it has two buttons, 'Close' and 'Ignore'.
It sometimes also then gives me some kind of print error dialog box.
FireFox Version: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:220.127.116.11) Gecko/20080702 Firefox/18.104.22.168
Microsoft Windows 2000 5.00.2195 Service Pack 4.
Here's a screenshot of the error message: http://i14.photobucket.com/albums/a302/AiXeLsyD13/WERIDERROR.jpg
Is this a virus? Trojan? Worm? Program error? None of my google searches for RSINST~1.EXE are turning up solid info. Should I be looking for something else?
Any help would be greatly appreciated!