You need to tweak a registry setting in order to allow connecting to a windows share that is shared on a windows 2000 or xp machine from a Windows 7 machine. Here is the technet article that describes the registry setting that needs to be changed:
http://technet.microsoft.com/en-us/library/cc960646.aspx
Here is the relevant part from the post:
LmCompatibilityLevel
HKLM\SYSTEM\CurrentControlSet\Control\Lsa
Data type
Range
Default value
REG_DWORD
0–5
0
Description
Specifies the mode of authentication and session security to be used for network logons.
Value
Meaning
0
Clients use LM and NTLM authentication, but they never use NTLMv2 session security. Domain controllers accept LM, NTLM, and NTLMv2 authentication.
1
Clients use LM and NTLM authentication, and they use NTLMv2 session security if the server supports it. Domain controllers accept LM, NTLM, and NTLMv2 authentication.
2
Clients use only NTLM authentication, and they use NTLMv2 session security if the server supports it. Domain controller accepts LM, NTLM, and NTLMv2 authentication.
3
Clients use only NTLMv2 authentication, and they use NTLMv2 session security if the server supports it. Domain controllers accept LM, NTLM, and NTLMv2 authentication.
4
Clients use only NTLMv2 authentication, and they use NTLMv2 session security if the server supports it. Domain controller refuses LM authentication responses, but it accepts NTLM and NTLMv2.
5
Clients use only NTLMv2 authentication, and they use NTLMv2 session security if the server supports it. Domain controller refuses LM and NTLM authentication responses, but it accepts NTLMv2.