My two penn'orth:
Most of this is gained from 20 years of being a PC user, by the way, not necessarily a PC technician, so it MAY not be 100% correct in terms of terminology, but I think the main thrust of it is accurate.
14gig free (if I read it right) is still LOADS of space so I don't think that's the issue.
No other symptoms, apart from loss of performance? No dodgy 'dialer' programs in the process list?
Is it possible that your pc has suffered from having software installed and then removed over a period of time? If this happens, often the removal programs don't tidy up after themselves quite as well as you would like.
Some of this is good practice (don't remove something that another program might be using) btu some of it is just sloppy programming.
What this causes (over time) is a large number of program components (typically dynamic link libraries or dll's) are left in the 'load me at startup' area of the registry, and are loaded into memory when the computer boots up.
Anything that is pinned into memory reduces the amount of memory that the programs you actually want to use can see.
Less memory means that your pc has to use what is called 'virtual memory' which is an area of disk made available as temporary storage, but it's MUCH slower to read from/write to disk than to access the RAM memory.
See if anyone has a decent registry cleaner you could try? You do need to exercise caution when cleaning the registry, but it can find some 'widows and orphans' left from old software, which may be one of the root causes.