I am planning to buy a new DSLR nikon d40 after reading the forums. I have old canon 500N SLR. will i be able to reuse the lense on the D40 ( even if permits only manual focus) ?
Unfortunately the answer is probably not. There is an article
on Ken Rockwell's site which describes putting Nikon lenses on Canon bodies, i.e. the reverse of what you are proposing. In it he states "Canon cameras have a shorter distance between the lens flange and film plane than Nikon. Therefore there are a couple of millimetres left over into which a metal mounting adaptor can fit."
Thus even if you found an adaptor which would physically allow a Canon lens to be mounted on a D40 it would not achieve focus at infinity.
A similar problem arose when Canon moved from its old FD lens system to the current EOS system. You can still buy adaptors to fit FD lenses on EOS bodies but they won't focus on distant objects. Might be worthwhile for macro work as there are some very cheap bits of high quality glass out there but not for day to day use.
The D40 is a fine choice but, and here I apologise in advance as this is a Nikon forum, if you want to retain use of your Canon lenses have you considered the EOS 350D/Rebel XT body? Gordon has a 350D review
on this site as well as the D40 review
you have probably also read.
Good luck with your search.