I've installed WSS3.0 in Windows 7 64 bit. I am not able to open Central Administration. I get HTTP 500 Internal Server Error when I try to open Central Admin page.
You're not allowed to install WSS 3.0 on Windows 7 (this ability is added only from SharePoint 2010)
Please, read this MSDN article:
Supported Operating Systems: Windows Server 2003;Windows Server 2003 Service Pack 2;Windows Server 2003, Enterprise Edition (32-bit x86);Windows Server 2003, Standard Edition (32-bit x86);Windows Server 2008
You should use virtual machine or smth like this.
WSS 3.0 (SharePoint 2007) does not support installation on client operating systems. You need to install it on Windows Server 2003 or 2008.
EDIT: Bamboo solutions does have a setup helper that allows you to install WSS 3.0 on Vista or Windows 7 but I don't know what limitations there are or how well it may work. YMMV.
As stated earlier WSS3 will not work on anything but a server edition of Windows (2003/2008). If you're looking to play with SharePoint on your desktop I'd suggest grabbing your favorite virtual platform, setting up a Server OS there and installing SharePoint; or the new SharePoint Foundation 2010 will install on Windows 7.