Microsoft’s last update to Outlook Express for Mac was in 2002. OE v5.x runs only on Mac OS 9 and on *Mac OS X systems that can still run Mac OS 9 programs.* If your Macbook cannot run both OSes you will need to visit Microsoft's website to see if there is a work-a-round since it's their product.
Microsoft announced last summer that the next version of its Mac Office software (scheduled to arrive by the end of this year), will include a version of Outlook written specifically for Mac OS X.
And which Mail software are you planning to use in OSX 10.5.8 ?
OS X Mail ?
Thunderbird ?
Entourage ? Outlook ?
Entourage 2004 or 2008 can import from Outlook Express v5 (and an enormus number of other formats). Even the demo version of Office could be used to get them into a mail database on OS X. Then you have 30 days to work out how to get the stuff into your target Mail application.
It'll certainly be do-able, perhaps going via Thunderbird, it all depends on your preferred mail app, if it can't import from anything, then you're between a rock and a hard place.