Outlook 2000 very slow

Three of our users are complaining that their Outlook opens really slowly or sometimes don’t open at all.
I’m not very technical and just want to ask a few questions. There’s something like 3000 items in their inbox, according to me that must slow things down. Am I correct in saying that? And then what can they do to those items, because they want to keep a record of emails sent and received from clients. What will be the best way of dealing with Outlook to make it as efficient as it should be? And please tell me what I can check to make Outlook more efficient or the answers I can give them that will motivate them to keep their outlook clean.

Many thanks!! Really hope someone can help me.