Question 3 - How often should I buy a new computer?
If your current computer has Windows XP as the version of Windows installed, you should buy a new system now. Windows XP is no longer a supported operating system and is prone to infection from malware and virus files.
For business users, I suggest that you buy or lease a computer every three years. As fast as technology changes and as much work as a computer used in business gets, to be dependable, three years is a good rule of thumb.
For home users, a new computer system can wait until around 5-6 years. It depends greatly on the user and the amount of use and desire of what type of use the computer will be for. See Question 1 in the FAQ for what type of systems I suggest.