Apart from the above solutions there are other ways to do it, and to do it for all txt files in one go.
To maximize Notepad when double clicking a text file, in explorer:-
1) select 'Tools' > 'Folder Options' > 'File Types'.
2) Click the Files Types folder and look down alphabetically for "TXT Text Document".
3) Select it, click Advanced > open > Edit and replace the "application used to perform action" box with:-
cmd.exe /c start /max C:\Windows\notepad.exe %1
For Start > Run you can use this:-
cmd.exe /c start /max C:\Windows\notepad.exe "C:\your dir\yourFile.txt"
Information about all this is here cmd - Command Shell
I have XP Professional SP3, 32 bit.