So are you not using Windows 7 then? Previous versions of Windows do not (or did not) provide the functionality to handle TRIM correctly which means that the SSD will get slower as it is used not faster.

Have you had a look at some of the "handy hints" sites as to what you should and should not have enabled / disabled with SSDs? Some of them are important as they can significantly shorten the life of the SSD.

And have you tried one of the performance tests to see what the read and write speeds being achieved are?

No, it's on XP.

and it's a hybrid drive, a normal 7200 HDD with 4GB ram for the frequent stuff.

I'll have a rummage around the web.... ta