I suspect Cubase wants to run ASIO drivers. Try using ASIO4ALL.
Other things to try,
Audacity instead of Cubase as a check.
Then unistalling the Allesis and running on the internal soundcard. Won't be brilliant but should not cut out.
This is old advice but has worked before. Clean out everything to do with the AI and then disable the internal soundcard (AC97?) best done in BIOS if you dare, if not in Device Manager...Then re-install the AI.
How stuffed is it? I had a laptop with a 20G drive, 1/2G of ram and an 850 MEGAHertz processor that would record 2 24bit 44,1 tracks of Samplitude 8se and playback about 8, os was XP pro.
Extrapolate that to your machine and even if you only had 2 G left it should smoke Cubase. In any case you have 8G of ram and virtually everything is loaded into ram anyway, AFAIK the hard drive just "mops up"?