Odoo is the world's easiest all-in-one management software. It includes hundreds of business apps:
CRM | e-Commerce | Accounting | Inventory | PoS | Project management | MRP | etc.
As some others already pointed out, openERP is not too quick on Windows. When you run openERP 7 on Ubuntu, it is much more responsive, even when the Ubuntu-machine is a VM.
And it is not too difficult to install openERP on Ubuntu, there are various tutorials here and elsewhere on the internet.