I've got it running on Windows 7 and it runs ok here too Perhaps the local configuration options are set different to your old machine? Under Advanced > Preferences in the BOINC manager you should be able to set the % CPU and other things which would affect it.
100% CPU time would be the fastest at crunching, but you're going to be running your processor at full load all the time then (which may not be a good idea if you don't have very good cooling). I keep mine at 70%, as that's still pretty fast but the PC never gets too hot
You could try setting the max cpu limit at something like 60%, as that would still run really quite cool but be an absolute beast Even if it just crunches when idle it would churn through results.