Working with a used Sony VAIO Desktop PC, OS XP-Svc Pack-3.
It is Partitioned into C: 15gb and D: 40gb. No working files or folders on the D:

Desire isto remove the Partitions effectively reformating back to a full accessible drive.

Original MS-XP System Disk is available for reinstall if necessary. Do Not know if Sony VAIO's require any specific system files for full operations.

All HELP will be appreciated. And let's go with basic instructions step by step, Please!

You can either start in DOS or open a DOS window from within windows. It will look like a black box and often called a command line.

The tool your looking to use is Fdisk. It is a utility that came with your system.

Here is a site that explains it and how to use it.

FDISK.COM fdisk info

If the OS installed on C: is working then you can simple format the D: drive from within windows.
After that it's your choice, but as D: will be freshly formatted you could simply stay with the two drives.

Of course if you have the XP CD then a simple rebuild would easily restore the system to one drive operation.
In the installer, after booting to the CD, you delete all partitions then format and continue from there..

If it's an older system I'd be inclined to go for the rebuild option.
You will likely need some drivers, but they are all available from the Sony site..