IIRC, you need to be using IE5 or IE6 and Java SE6 (Update 6 rings a bell???) or earlier, all other versions of Java must be uninstalled, and this was with the final release of 3100 software... Earlier versions required J2SE 5 and were a little version finicky too.
I could never get this to work in Vista or 7, but used to use a WinXP virtual machine. It is really tough to get it to work if you have any IE "security patches" applied, anything within the last couple of years breaks the usage of any Java prior to SE7. I don't know that I can help much more, it's been 6+ years since I have seen a 3100.
Remember that this product was discontinued in around 2002 or 2003, so it is at least 13 years old... well beyond it's expected serviceable life of 7-10 years, you should seriously consider moving to a newer Mitel system, the MiVoice Business platform will support most of the IP phones to help minimize the initial investment.
