Two things occur here: First, mySpace is horrid and will always load slow; Facebook should be OK but uses a lot of CPU intensive Flash ads.
One thing you can try is to open the Task Manager, leave it open while you browse around.
Hold CTRL and ALT, press delete key ONCE. Click the Task Manager button.(There may be other ways to open it, this is just what I've always done.)
Take a quick scan of running programs, select the applications tab. FYI if a program freezes you can select one and hit the "end task" button.
The Processes tab will show you some real-time stats. In the CPU or Mem Usage columns, click the header (the boxes with the actual words "CPU" or "Mem Usage") once to order the entries by lowest numbers, then again to order the numbers by highest (what's using the most.) This will give you an idea of what's using your memory and CPU. The numbers are in K (kilobytes,) 1000 K = 1 MB. It's not unusual for some programs to use 100K+, but simple addition will tell you if this is exceeding physical RAM.
If it's exceeding physical RAM, Windows will increase the system Page File and use disk space as "virtual memory." This definately slows down overall performance.
Under the performance tab you will find a real-time graph of CPU usage and page file usage. An "average" CPU usage should be no higher than 5-20%, if it's topped to the ceiling something is hogging your CPU. (Although it's not unusual fora "spike" to occur here and there as you launch programs, etc.)
I notice the CPU on my comp tops out when browsing Facebook, the ads there are CPU hogs. (2.8 gHz)
The Networking tab is really only relevant if you have a router or other network setup, but if you do, you can have a look and see. I'm on satellite Internet with a router and shared connection here, this is almost always a flat line near/at zero with small "bumps" as the XP system polls the network.
Putting all the bits together you might find a weak point that needs to be looked at, if you have 768 MB of RAM, I'd throw a chip or two in there, it's pretty cheap these days.