One of the biggest problems with Windows, and what has many people jumping ship for Macs, is that it suffers from clogged and infected registries.
Registry Clean Expert scans your Windows registry and uncovers incorrect or obsolete information in the registry which in theory should speed up your machine. However, don't get too excited because whatever is slowing down your PC may not even be related to your registry - it could just be a case of a trojan or malware that's infected your hard drive. Also be careful because any changes you make to the registry, can destabilise your system. However, Registry Clean Expert does give you the option to rollback any changes you've made with the backup/restore function.
The most useful function for me though was that you can also edit an manage BDO items in your startup sequence. BDO items are those items which launch themselves when you turn on your system. They are usually entered in your registry too and if your system is taking increasingly long to start up, then you may well find that it's the BDOs that are causing the problems.
While it may not always fix your registry, the BDO cleaner will certainly speed up your startup's and shutdowns.