My biggest problem moving from XP to W7 has been that Outlook Express is no longer supported. My computer shop said Live is useless, and recommended using Mozilla Thunderbird. Most people making the transition probably just walk away from their old e-mail archives as the migration is reported to be somewhere between ridiculous and impossible.
I won't say the transition was painless. Initially T'bird was ultra slow due to interference from my anti-virus software. Migrating old email files from OE to T'bird was painful (one must put on one's Administrator mask and dive into hidden directories to retrieve files from unlikely-looking directories). But it eventually worked.
But now the problem is my wife is off her ancient XP and on my new W7 laptop. And we upgraded the XP to a solid state hard drive a few months back. So maybe that machine can be upgraded to W7 and I can get my laptop back!