SQL Server 2005 includes some native encryption procedures which you can use to encrypt the data. However decrypting the data on the SQL 2000 server or the Access database will be harder as they don’t include the same encryption procedures. (SQL 2000 doesn’t include any native encryption procedures at all.)
If you have the option of setting up a VPN connection between your store and the co-lo network that would be the easiest way to encrypt the data as all data sent over the VPN would be encrypted.
There are some third party DLLs which can be used to encrypt and decrypt data within SQL Server 2000. Your best bet would be to find one that uses an encryption level which you are happy with and install this third party DLL on both the SQL 2000 and SQL 2005 systems (so that you can use the same code on both systems) and use this DLL as an extended stored procedure to encrypt and decrypt the data.
If you don’t have the option of installing a third party DLL on the hosting companies SQL Server you may want to look to upgrading your local system to SQL Server 2005 so that you can use the native SQL Server 2005 encryption on both ends of the system.