Memory leaks are caused by buggy software and the cure is to fix the bugs or get updated versions with the bugs fixed.
Windows slowing down, however, can be caused by: - your registry getting full of crap - your hard disk needing defragmenting - your sytem full of spyware and adware
Outlook Express may be slow because your mailbox is getting big. Look for file mailbox.pst and see how big it is. Consider archiving or deleting old messages.