cant. virtual pc addons are not available for linux.

you will have to use samba and share them over a network or something

Then it is bad because i need to backup all my files and database when i am in ubuntu and use them in xp again. If i can't do this i think i should install ubuntu without vistual pc and make dual boot.
Is it logical?

just back them up and send them over the network using SAMBA.

i'll have a look into SAMBA. Thanks

Well I this is a workaround I had done while using VirtualBox which had Ubuntu (the OS in my Virtual Machine), on my Windows XP laptop.

The folders in Windows XP on which network sharing is enabled, are accessible in Ubuntu.
All you have to do in your Ubuntu box is:-
Go to "Connect to Server", In the first drop down select "Windows Share (smb)", put the IP Address of your Host OS that is IP Address configured for your LAN card in XP, enter the "share name" which should be the name of your shared folder in Windows XP and click on Connect.

So you shouldn't need to take the pains to install and configure the samba server. Also if the architecture of VirtualPC is anywhere near VirtualBox I do not think you will be able to directly access the shares of the Ubuntu machine from Windows

Note: -
Ubuntu 8.04 has a bug which will not allow you to connect to a Windows share unless and until you specify a share name. in Ubuntu 7.10 this bug was fixed, so if you would not enter the share name in the Connect To Server dialog, it would show you all the available shared folders on the given machine. I do not know if it has been fixed in 8.10 .

Also forgot to mention, if you are going to take such a drastic step then instead of Virtual PC you could try VirtualBox, guest additions for virtual box are available for almost all major OSes.