unlike Spunk who failed to answer the Q, no you cannot change the speed.
Download and install TweakUI (free), it has an option that allows the user
to adjust the slideshow timeout.
Just open TweakUI and go to...
Explorer / Slide Show...
Windows XP PowerToys / free download
http://tinyurl.com/36n
Correct URL:
http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx
...be sure you get the correct (32-bit vs. 64-bit) download.