If you slave your old drive off the new one
after you have installed XP on the new drive, you can simply import your mail into Outlook Express on the new drive.
Before you dismantle your PC, you need to do the following:-
Open Outlook Express and select Tools|Accounts
Select the Mail Tab
For each email account listed, click it and choose Export, saving the file to an easy to remember folder.
Choose Tools|Options and select the Maintenance Tab
Click store folder.
Write down the path given which is the path to your Outlook message store.
When XP is installed you will then need to Import the mail accounts (from Tools|Accounts) and Import the mail store (from File|Import|Messages). Dont forget that the path to the message store is now probably on D: rather than C:
I hope that makes sense