Depending on the speed of the computer you're using, how much you're using it for normal tasks, and how complex of work unit you were assigned, it can take a couple of days for you to show up (your client must send in one WU before you'll appear in the list). If you're not using the command line interface (CLI) client, but rather the GUI one, take a quick scroll through the FAHlog.txt file in your Folding@Home folder to see if you spot anything about "simulation instability encountered" or similar. If so, it's most likely not able to process work due to the system being overclocked too much; if that is the case, you'll need to back down slightly to make your system stable so F@H will be able to run. Also, check the Task Manager to make sure that another process aside from FahCore_??.exe is not using up all of the CPU power without your knowledge.