Well such scans take a long time to scan, especially if you are infected or if your system specifications aren't good enough. With good enough I mean at least 1 GB of RAM and at least 150 GB of Hard Disk space.
Now, what you can try is defragment your PC so it runs faster and also scans faster, and you can scan for viruses, not only spywares and adwares. ( To do that you can just post a hijackThis log here ).
So how to defragment, go to MyComputer>Local Disc C:/ ( or what is your letter ) then right lick and Properties>Tools tab and find defragment. You can first analyze to see if it's needed to defragment and when you click on defragment leave it so the process is being done. Warning, it also takes much time, on my PC for about 15 minutes and I do it every 2 months.