Not a whole lot to add. Was travelling all day yesterday.
Memory (Ram) will tell us a lot. You also want a swap space at least the size of your ram.
When is the last time you rebooted the PC? If its been a long time, this may help on windows.
If your hard drive is completely full, this can be the cause of virtual memory problems. Even with 500MB left, if could be so fragmented that there isnt enough space to swap programs out of memory onto the disk. If this is the case, find any non-essential files and delete them, or better yet, back them up onto an external device and then delete them, this way you have a backup incase you need it.
The constant running sounds like an issue where there isn't enough memory to handle the files that are open, and the processor spends all of its time switching between processes and loading them rather than running them. Depending on that amount of RAM you have, you might be able to add another stick for super cheap and fix the issue.
the MSCONFIG suggestion is a good one, if a lot of programs are starting at boot time, they can take over the whole system and prevent anything from running.
If you get a chance to post up the RAM, disk usage, and processor you are using, we should be able to get this fixed for you.