If you are using the free open source Mozilla Thunderbird software for email, do not, repeat not, upgrade to the latest version i.e. Thunderbird 3!
I've had terrible problems since trying yesterday to upgrade from Thunderbird 2, with Thunderbird 3 freezing, not responding, hanging, taking several seconds to respond to every click or keystroke.
Still trying to figure it out but as far as I can see, Thunderbird 3 defaults to trying to synchronise all email in all your folders from all your e-mail accounts - which is a bad idea and a half (this default is only meant to operate for newly added IMAP accounts, but in my case it's doing it for all my existing accounts).
That means that the moment you open Thunderbird 3, it tries to download all your Gmail etc. All of it. Talk about taking up computer memory and processing time.
I've tried to untick most if not all of the mail folders (menu Tools > Account Settings > Synchronisation & Storage > Advanced button, untick everything but Inbox & Sent, and even those if you want, then OK and OK again).
But even the process of unticking is proving to be a nightmare, with every untick taking 10 seconds or so to "take".
And it's still downloading the ticked emails… maybe I should have unticked them all, but there we go, I'm going to let it carry on trying to do its stuff for the rest of the day & see if it's faster after it's got everything downloaded for offline use. Even unticked folders seem to be downloading, not sure why, once I make sure the OK has taken (which is literally taking hours) I'm going to kill Thunderbird & restart it.
UPDATE: it took about 10 hours to do, as mentioned it wouldn't let me OK the UNticking of folders until it had downloaded everything. So my tip is, if you are going to upgrade to Thunderbird 3, do it just before you're going to sleep or go out for 12 hours. Other apps on my computer were usable, just not Thunderbird. Then, as soon as you can, go change the Sync & Storage settings to untick what you don't need.