Some people keep it on all the time especially if they use it a lot.
Some people turn it on only when they're working on it and then shut it down.
You really won't affect the life of the PC one way or the other. Just make sure you've got everything plugged into a power strip to protect you from surges - that's a good idea. I have a win 3.1 that was upgraded to win 95 with a pentium processor. You can always direct the installation of a program to a drive other than your C drive. When the Install Wizard gives you the directory it's going to install in, it will ask you if you want to go with the default or choose a different directory.