Cant Copy files between Windows Vista and XP macines

I have many clients i connect to to download updates etc to. When I connect
using Vista Business at my end Xp or SBS 2003 at their end and try to copy
large files (15-20 mb) one of the following the following happens:

1) I can select the file to copy but there is no paste option enabled. No
way to transfer it?

2) I can copy and then sometimes there is a paste option but when I paste it
a message comes up "Preparing to copy..." and waits for around 5 - 10 minutes
before starting to copy and then takes another 10 - 15 minutes. OR the
copying never starts, no matter how many times you try to paste the file.

Thanks
PhlipC