Some thoughts
The usual purpose of partitioning the HD is to put your data on a separate drive for protection and ease of backup. It's also done a lot in a corporate, managed environment for ease of remote mgt. over the network. Also, it means that you can upgrade the operating system and applications without endangering the data.
That said, since XP tries really hard to put all your data under "documents and settings", the value of the partition can only be realized if that folder is written to the D: drive. You don't say if it is. That's not the normal configuration. If it's not on D:, then you are cramming everything into the C: partition of 14 GB.
I don't know for sure, but 14 sounds low for the operating system and the applications. On a home-use computer, it's often not partitioned and I would not do it. On my home desktop unit, I'm using a little under 8GB for "program files" and "windows" alone.
OK, now for the speed issue. If there is a lot of writing and re-writing to the HD, it will become, to some degree, fragmented. This is a tekkie way of saying that the areas where files are written are no longer in nice, adjacent, compact chunks. Rather, the data for a single file, if it's large, will be all over the place and the HD has to keep jumping around to read the whole file, let alone write it. Windows provide for de-fragging, but PLEASE fully back up your data before you do it. Or get an expert to do it. There are third party programs which do it even better.
Speed issue again - the more likely candidate is physical memory (At the risk of insulting the more informed readers, memory and storage are not the same. Storage is on the HD, memory is in the chips and is constantly being reused at the speed of light, nearly.) When there is not enough memory to hold everything the computer is trying to do at one time, Windows writes large chunks of the memory to the HD. We used to call the "paging". This is very slow. I don't remember the numbers, but I think it's no exaggeration to say that memory is at least 100 times faster than the HD. If you don't have a lot of memory (I won't run XP on less than a GB of memory) then you have to try to keep its use clean and efficient. There are two primary techniques. First, only load into memory what must be loaded. It's OK to take a little longer to start a program if it runs well after it starts. Don't keep in memory what is rarely used. Look in your
c:\documents and settings\username\start menu\programs\startup folder to see what is getting loaded on log-in. Secondly, close each application when done with it before starting another. Sometimes you want to run two or three at a time, but often you don't really need to.
The first step in a solution is to decide what you want to do about the partition. The second is to take it to someone who knows what he or she is doing and pay them a fair price for their work. Work interactively with him/her on cleaning up your startup stuff. Do not attempt this at home if you are a self-professed novice. Good luck.
PM me if you need more consultation on this.