If the file is a JPG, already selecting it and copying it elsewhere will not make it lose definition. The file remains the same (i.e. Copy "this whole thing" (the file, the image, not a screen capture of it) then Paste "this whole thing").
Where you might be losing definition is if you are copying the screen image (such as via some screen capturing software). What you see on the screen may not be all the data, in all of its glory.
(By the way, MSDN says that without altering a JPG file, then multiple saves will not make you lose detail. They also say that simple image manipulation, such as whole 90 degree rotations, or image flipping, and you will also not lose detail. It is other alterations that will probably cause loss of detail. Oh, also they stress that the image must have height and width pixel dimensions exactly at multiples of 16. But that is getting too nerdy, sorry.)
Paint does not have provision for adjusting how much compression (more compression = smaller file size = more loss of detail), but
IrfanView does.
From what I can tell, from some quick tests in comparing resulting JPG file saves, it looks like Paint's compression is set at approximately 75 (1 - 100, where the lower the number results in more compression, and more loss of detail) when comparing Paints JPG file save, to IrfanView's 1-100 values.
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By the way, the value of 100 is NOT 100%, or "lossless"
All JPG format images have some loss to help in the file compression.
There is a new JPEG2000 format, that saves files with the extension JP2, that has the ability to be "lossless." IrfanView can read and write JPEG2000 (JP2) images.