If you downloaded the VHDs that Microsoft has for learning 2010 then you
won't be able to run them on Vista 64. It requires Server 2008 r2 and
Hyper-V. If you have Windows 7 64 you can boot to a single VHD, but the
downloadable one contains two VHDs (one a domain controller and the other a
server), so again you would need Hyper-V.

You can build your own VHD from scratch on Vista 64 using either VmWare or
Sun Virtual Box, but 4gig of RAM is cutting it pretty close for a virtual
2010 install. You should probably have at least 6 gig if not 8gig even to
build a learning environment.

Ideally you should have Windows Server 2008 R2. RAM is good enough for a
learning machine. You can't run this using Microsoft virtual PC because VPC is
32 bit.

You may use VMWare workstation if you can afford.

I am not sure about Vista 64 bit, but Windows 7 does support SharePoint 2010
after playing some tricks. You will find ample knowlegde about it on the web.