memory is static, meaning the size does not change when referring to computer RAM. Programs use certain amounts of memory when they are doing their applications, then the memory is allocated back.
Hard Drive space is a different story, it is ever changing, from programs being installed taking up drive space or applications using drive space as cache files the size fluxuates.
You can check the hard drive space like Ann showed, and you can see how much memory your computer has by right clicking on my computer then properties.
You can purchase memory for under 20 bux nowadays so it's always a good investment and will make your computer run better...
But since ya said it's a HP with windows 98 there really isn't much you can do to make it faster until you get a new computer