I had Windows 7 installed on my Laptop's C-Drive, but lately it wasn't working well, so I re-installed it on the D-drive. It now seems to be working better, but none of the old programs work anymore. What should I do to get my computer to "like-new" shape. Should I reformat the C-Drive, and then re-install the programs and paste back the files that were there? What should I do with the D-Drive, which has both the new Windows installation, and old files and programs? Manually delete the folders of all the old programs?
Since you installed a new instance of Windows 7, all your programs will need to be reinstalled since they must all write to the new registry.
I will assume that you have only one disk, but with two partitions, but even if you have two disks, I would do the same.
First, backup your data if you have not already. You are doing a lot to your system, and if you make a mistake, you will be glad you did.
Second, I would run some kind of serious disk checking utility to make sure your initial problems were not hardware-related (it does not sound that way, but now is the time to find out). I have professional tools to do that, so I am not sure what to recommend here. You could ask that in another question.
You can then do either of the following:
The average user should only have a max of 2 partitions on a single physical hard drive. This is so that disaster recovery is simpler and much less expensive.
The OS and all used programs should be installed into the C:\ partition whereas all data should be put into the D:\ or remaining partition. Thus, if the OS kills itself, your data is safe; at least most of the time. This method of partitioning also allows you to image C:\ partition so that if your OS does kill itself, you can reimage the partition and be up and running in as little as 20 minutes.
I am assuming that there is only 1 physical hard drive.