Most PC sellers let you return a bad system within 30 days. The company probably will ask you to attempt to fix a problem by telephone with a tech-support person. If you buy a computer from an Internet vendor, you'll probably have to deal with the manufacturer if the system goes bad. Find out who pays the shipping in case the system has to go back. Of course, you won't know there is a problem with your new system until you open the box, set it up and turn it on.
Computers tend to become obsolete fast.
First, keep the boxes and the packing material for at least the first 30 days. If you have to ship the computer or monitor back, do so in the original box. Next, document everything in writing. If a problem develops during that critical 30-day period, make a written note of it. Know in advance the vendor's return, exchange and restocking-fee policies.