Unless you are 100% sure of what you are doing, leave it well alone. Even the most reputable registry cleaners and/or optimizers make mistakes.
A registry cleaner is a type of software utility designed for the Microsoft Windows operating system whose purpose is to remove redundant or unwanted items from the Windows registry. However the necessity and usefulness of registry cleaners is a controversial topic, with experts not agreeing on their benefit. The problem is further clouded by the fact that malware and scareware is often associated with utilities of this type.
Registry Cleaners are software utilities that attempt to remove configuration data from the Windows Registry that is no longer in use or that is unwanted on the system. Such data may include information left by software that has not been uninstalled completely from the computer, information that is no longer of use, or settings required for the operation of malware. A registry cleaner scans the registry, and attempts to pick out the unnecessary values in order to delete or repair them.
Most notably, critics say there is no reliable way for a third party program to know whether any particular key is invalid, redundant or neither. Poorly designed registry cleaners may not know for sure whether a key is still being used by Windows or what detrimental effects removing it may have. This has led to examples of registry cleaners causing loss of functionality and/or system instability, as well as application compatibility updates from Microsoft to block problematic Registry cleaners.
On Windows 9x computers, it is possible that a very large registry could slow down the computer's startup time. However this is far less of an issue with NT-based operating systems (including Windows XP and Vista) due to a different on-disk structure of the registry, improved memory management and indexing. Slowdown due to registry bloat is thus far less of an issue in modern versions of Windows. More importantly, however, the difference in speed due to the use of a registry cleaner is negligible: rarely do they remove more than a few kilobytes from the total size of the registry. In fact, technology journalist Ed Bott has claimed that no one has ever successfully managed to measure any significant performance increase from the use of a registry cleaner. Even so, there are reviews showing different performance benefits for various registry cleaner products.Any potential user of a registry cleaner must thus balance a probably negligible performance increase against the possibility of system instability. A safer and more measurable approach to registry performance is to defragment the registry files using a Microsoft-supported tool such as PageDefrag.(NTREGOPT is another free util that checks and modifies only the placement of these files on the hard drive to optimize it and not the content of the registry themself.)
Thanks so much for the fast reply - people keep telling me that this is why my system runs slow - I know I have to upgrade memory - which I intend to do -
An increase in RAM will give you the biggest boost in performance.