Most IT pro’s probably don’t have a problem in getting access to the Windows 7 operating system installation sources because they can access them through their corporate volume license agreement and/or MSDN or TechNet subscription. But unfortunately there are still some out there who do not have such easy access to these sources.
For the use of Windows 7 on a primary device (the one that one uses on a daily basis) an official version of Windows 7 that comes either from the Corporate Volume media source or a separately acquired copy or pre-installed with a new computer must be used.
But if you need Windows 7 just for evaluation purposes Microsoft provides the following 2 options:
Windows 7 Enterprise 90-day Trial
The Windows 7 Enterprise 90-day trial program allows you to download the Windows 7 Enterprise 32 or 64 bit installation sources which can be used for evaluation purposes during 90 days. The Windows 7 Enterprise 90-day trial can be downloaded from here. Note that prior downloading the sources you must fill in a form which also includes a question about your occupation. If you select a profile that does not fit to this program, you don’t get access to the sources.
Microsoft Windows 7 90-Day Eval VHD
This is a preconfigured virtual machine set contained within the Virtual Hard Disk (VHD) format. To run Windows 7 within the VHD you must have access to a system that has Microsoft Hyper-V installed. The Microsoft Windows 7 90-Day Evaluation VHD can be downloaded from here. I recommend to read the Readme prior downloading the sources.
Finally let me also mention the Windows 7 Test Drive which provides access to a Windows 7 client that runs remotely on a Microsoft hosted virtual infrastructure.