You can easily upgrade the hard drive and the memory to 4GB if you have less than that. You might be able to upgrade the CPU that's only if your motherboard model supports something better. You would need the repair manual to see what CPUs are compatible with that motherboard and it would require you to take the whole thing apart. Even then it probably wouldn't be worth the effort
. As for the GPU that's the best you are going to do.
So what kind of PC do you have and what's wrong with it