Connecting via UNC to XP boxes

I run W2KPro boxes on my network. Lately I have started getting some boxes with XP Pro on them. I browse the network a lot using UNC (where I just type in the pc name at the run box). When I am on a 2000 machine I am unable to connect to an XP box. Error states that network path can’t be found. I have no problem connecting from one XP box to another XP box. Anybody got any ideas? Thanks.