Cannot access Windows XP PC from windows 7 PC in the same network [ SOLVED ]

In Windows 7 box, do the following :

1. Type secpol.msc in RUN.
2. Select Local Policy -> Security Options.
3. Then select " Network Security : LAN Manager authentication level "
4. Edit that policy and select "Send LM & NTLM - use NTLMv2 session security if negotiated"

Done.