What program are you using on the PC? Thunderbird? Outlook/Outlook Express? Outlook2Mac is what I suggest to clients who are switching. Thunderbird may have its own package similar in functionality, but I found this blog post that discusses how migrating Thunderbird on PC to Mail.app on Mac works. Last post in an archived MR thread shows another way of going from Windows Mail in Vista to Mail.app by means of Thunderbird for Mac.