Hello Garish,
If you are running the UD Windows client, then it is using only one core. You still benefit a bit from the other core, because the OS and any application programs you are running have two cores to run on.

If you are running the BOINC client, then you have the option of setting your profile to run 2 threads to take advantage of each core. Just remember, this may not be a good idea if you are hyperthreading on a single core, since that will try to shove two giant scientific programs into one L2 cache. Some power users have multiple computers. When they get a new dual-core Pentium, they change their profile to take advantage of it and then discover that their older machines are also trying to run 2 threads using hyperthreading. The solution is to create a single thread profile (call it 'Home') and a dual thread profile (call it 'Office') and assign the proper profile to each computer.