For starters, your OS actually plays a vital role when it comes to the performance of your system and this becomes very important if the OS is heavy and your system is old. In such cases, you feel that your system I becoming weaker by the day. In such cases, you should really make it a point to reconsider your OS preferences so as to suit your PC performance. The golden rule here is to go easy on your system and you will have to reconsider the OS options that you are provide with. To start with, Windows is heavy for your old system even though the latest version of Windows 8 is considerably easy on this respect. Nevertheless, it does not come near the ease provided by Linux. So the optimum choice for you is Linux and the various options that they give.
Puppy Linux is one of the most light weight Linux OS and you can really consider this option if your PC is very much weak and you would not have to worry about a thing if your PC is the one listed above as it has been reviewed as very good for personal use by the huge number of users it has got.
Well your options do not end there. You do have other options like Vector Linux. In the specific case of it, I would like to add a word of advice. If you want to choose between Puppy and Vector, then you should consider the former as you get more options from that.
Well then there is the always famous and popular Ubuntu. I need not have to explain the various useful features of it as you are very much aware of it. Though the case is so, I would like to add a thing or two. The important thing is that though Ubuntu is fine, it not as light as Puppy Linux.