This subject has been discussed here every few days since the NG started,
so there is a lot of valuable information already posted.
That said, you cannot continue with OE. It is not compatible with Win 7,
period.
As far as Windows Mail, it is not compatible with Win 7, but Windows Live
Mail was written to be compatible with Windows 7, and is as close to OE as
you will get. If you buy a computer today it will "probably" come with WLM
pre-installed. Outlook from Office 2007 is very close to OE, but does not
have NG reader built in.
I've used Outlook for mail for years, and like it. I used OE only for
NG's. Once I was aware that OE was gone, and while I was still using XP, I
experimented with several programs for NG use. I settled on Thunderbird
from Mozilla.
You have lots of choices and my advice would be to try some out while
you're still using the old OS. Win 98 SE? Holy crap.