Reinstalling Debian from a weekly snapshot CD (Lenny) is a good idea (download here
Or you can add the debian lenny repositories to your sources.list file, and update every package on your system.
# debian official packages
deb http://security.debian.org/ lenny/updates main
# deb-src http://security.debian.org/ lenny/updates main
#debain official packages 2
deb http://ftp.uk.debian.org/debian/ lenny main contrib non-free
# debian multimedia
deb http://www.debian-multimedia.org/ lenny main
Both methods will take a long time, since in both methods you need to download hundreds of megs of packages (in an iso or deb form, doesn't really matter). A clean install is always better, but the update should work too.
Of course you also can (and is advised) change the debian mirror server for your country. As you can see in the "debain official packages 2" section, there is uk in the address. You can choose the closest mirror server from this