" It shouldn't contain any extraneous data "
I am lucky to have dived into XP's registry after a clean install and SURPRISE!!! There is plenty of what can be called "extraneous" data or entries.
Please update this section of your claims.
The Microsoft Windows registry is a hierarchical database of system configuration data. Many of the programs written for Windows 32-bit and 64-bit platforms require data from the registry. This means it is used often, and some programs really like to hammer the registry. Running a program such as SysInternal's RegMon, which logs access to the registry, can show you just how much some programs make use of the registry. Even Explorer, the file browser built into Windows, makes heavy use and this can have wide impact on performance.
When Windows is first installed, the registry is relatively lean. It shouldn't contain any extraneous data nor is the data file bloated or very fragmented. Over time though this can change, especially when many programs are installed and uninstalled. When programs and other components are removed from the system they can leave behind data inside the registry.
Many programs have been made to clear out the data left behind by poorly uninstalled programs. The original, RegClean, was developed by Microsoft. A search on the web will show many results for registry cleaners, some sites touting their commercial solution as a cure-all for any Windows problems. Choosing a registry cleaner is best done on good advice, and forums are a great place to look and ask what cleaners people are using. I suggest you find a shareware or freeware cleaner, it's not worth paying for one when there are excellent free ones available. RegClean is available from Microsoft Download Centre.
After data is deleted from the registry, the space in the file used by that data is kept until it can be re-used by newly added data. If lots of very small bits of data are deleted the registry can become very bloated with 'holes' where no data will fit. This causes the registry to be larger than neccesary which in turn means it is spread over more hard drive space and hence slower to access. To solve this issue there registry compactors which can rewrite fresh registry hives without the holes. I use ExperimentalScene RegCompact.NET to compact my registry - with great success. My system very noticeably speeds up when the registry has been very bloated and I compact it.
Once the registry is cleaned and compacted it also should benefit from being optimized. This is done by rearranging the location of the registry hives as they are stored on your hard drive. Like normal file optimization or defragmentation this process will further enhance the speed of the registry. To optimize the registry use SysInternal's PageDefrag which also happens to defragment the virtual memory page file.
Following these steps in order will keep your registry nice and lean, speeding up your Windows experience.