There's also Opera Mini for Java ME. It reflows the websites for what screen resolution it's running at, usually quite well, and gives you searchable text. It's intended for featurephones but I once got a legacy version of it to run on an old laptop with Linux for fun with Microemulator [4].
Of course, an administratively simpler option would be to have the old machines access a server running the users' an entire desktop with a modern browser, PDF viewer and the like using remote desktop software. I arranged this kind of setup with a Linux server and a Windows client for a legacy workstation at work and it proved quite usable. Out of all the major remote desktop software that would run on old Windows (VNC clients, Remote Desktop, NX) the RealVNC client proved the easiest on the client machine's CPU in our setup.
If you had to absolutely make it happen from within the browser you could employ a VNC Java applet but that would be to the detriment of user experience. For security and privacy you can tunnel VNC over SSH.
[1] http://www.tenox.net/out/#wrp
[2] http://virtuallyfun.superglobalmegacorp.com/?p=3866
java -jar microemulator.jar opera-mini-4.4.31891-advanced-en.jad
Sorry for the mixed up sentences in my previous comment. I posted and then edited it in-between things and didn't notice the mistakes until too late.Allows for the testing of internal and external URLs with a number of systems including IE 6 on XP (but I don't see IE5 in the list)