1.  Why is my computer slowing down?

There are a number of reasons that could be causing the sluggish performance.  Here are a few:

a) Nearly full Hard Drive

If your Hard Drive is completely full, it is likely that many of your files are fragmented.  This is because as the hard drive fills up, there are fewer open blocks of space to write additional files.  The result, new files get written on different sections of the drive (file fragmentation).  Your computer takes longer to read fragmented files, since it has to scan more of the hard drive.   
  
Solution:  Delete unnessary files and run a file defragmentation/optimization program such as Norton Speed Disk.

If you're using Windows, there is a high probability your operating system is infected by various spyware program and viruses.   These prgorams run discreetly  within your system, and take up CPU and memory, slowing down your computer.  They usually find their way into your computer through your internet connection and are often hard to detect.  Viruses and Spyware can cause anything from a small drop in your computer's performance to bringing your machine to a halt.

Solution:  Use Windows Update to download the latest security updates and install at least one antivirus program and antispyware program to find and remove malicious files.

c)  Low Memory (RAM)

Everything that happens on your PC has to go through the computer's memory at some point.  So if you don't have a lot of RAM, it may cause bottlenecking when your computer needs to move a lot of information at once.  Typically newer software programs require more RAM than older programs, since they often have more features and better graphics capabilities.  If your computer could run old programs well, but is having trouble running new programs, upgrading your memory is the first thing you should do.  Its relatively inexpensive and can breath new life into your machine. 

Solution:  Add more RAM to your computer.  Make sure you get the correct type of memory for your machine.  Check the size, type, number of pins, and speed of RAM. 

b) Spyware & Viruses