Your profile indicates you are using 'Web Designer Pro'. As there is no such version, could you mention the exact application and version you have installed please?

First thing to check is to optimise all JPEGs (under Utilities)
Once done, save and close the program. When you restart and open your file, you will be working with optimised JPEGs in memory which will mean much less uncompressed megabytes for Xara to deal with.

However, 100 pages is still too many for graphical web sites by all accounts.