Opened doc in notepad-how change back?

[Granny]

I have no idea if this is the correct to for this since I do not know where
the error is coming from.
I have borrowed my sons PC and an error message keeps coming up; the only
option I have is to close the window. No other information is available; no
error message.
It is:

"The disk containing your index is almost full. Indexing will not continue
until more space is available. Please free some space or move your index to
another location."

I know absoultely nothing about the engine of this PC; so please be gentle!

Thanks,
Linda