I purchase one copy of the PC checkup online. Went online again and purchased second for wife's PC. Got the confirmation and a new serial number. However, when I try entering that new serial to download the product, it rejects, saying "invalid" confirmation. I need help, please!!!
The nature of your problem is not one that any poster here can help you with, sorry. Other than to ask ....
Do you have the installer from the first installation saved to the computer where it is now running? If so, you can copy it to a memory stick and then plug that stick into your wife's computer copy the file to that computer and run it to install the software.
If you do not have the installer saved and if you are sure you are entering the serial number absolutely correctly and still see the problem you need to email firstname.lastname@example.org and explain the problem to them. If there is an error with the supplied serial key they will be able to supply you with a new one.