This may take a moment to load and it should look like this:

You can see

2)  Find out some of the stuff that runs (in RAM) at startup.
a.  Click on the Start Button
b.  Click on "run" (right hand side).  It is possible that this doesn't display off of your start button.  It is a customization option.
c.  Type in "msconfig" where it says "Open:"
d.  Click on OK

e.  You will get a new window that looks like this.  Click on the tab that says "Startup"  (right side)

We can consider un-checking some of the boxes and this will reduce the amount of stuff that you don't need, but takes memory when you start up.

a.  Right click on an empty area of the middle of your Quick Launch bar (at the bottom)
b.  Left click on "Task Manager" near the middle of this context menu.
c.  You will get a window like this.  Click on the tab that says "Processes"
d.  Click on the header that says "CPU" (sorts ascending).  Click again on the header that says "CPU" (sorts descending).  This puts the process in order by which is taking the most resources.  

I look at this list if my computer has become very slow.  Most often it is my antivirus program doing something like a scan or update of virus signatures.

c.  You may get a pale yellow banner at the top of the browser page which you will need to click on to get the installation of the small active-X program.  Choose "Install the Add-on for all users ...",

then when the security window comes up, click on "Install"

I think you will also need to allow pop-ups for this website to get the web page that has the information you are looking for.
d.  You should bet a window that looks like this.

a)  Double click on "My Computer"  You will get a window something like this (depending on your "View")

b) Right Click on your "C:" drive and choose "Properties"

c) You'll get a window like the above.  The pink shows the proportion of Free Space.  The Capacity line (just about the pie chart) will tell you the total size of the hard drive.