A comparison of the difference between the two from Microsoft:
Q: How is this different from Windows
Defender?
A: Windows Defender detects and
removes known spyware only. It is not
designed to protect against the full
breadth of malicious software, and
specifically does not prevent viruses,
worms, Trojans, and other malicious
software from infecting your machine.
The new no-cost solution will be a
comprehensive anti-malware solution.
Q: Is Microsoft Security Essentials
designed to replace Windows Defender?
A: No but if you are running Microsoft
Security Essentials, you do not need
to run Windows Defender. Microsoft
Security Essentials is designed to
disable Windows Defender in order to
manage the PC’s real-time protection,
including anti-virus, rootkits,
Trojans and spyware.
C:FILE.TXT refers to FILE.TXT in the current directory of drive C:
C:\FILE.TXT refers to FILE.TXT in the root directory of drive C: (C:\)
They are not the same.
Edit: Command Prompt Example:
Windows remembers a current working directory for each drive. Say you are working in directory C:\UTILS and then you change to drive F: and then to directory BIN on F::
C:\UTILS> F:
F:\> CD BIN
F:\BIN>
At this point the current working directory for C: is still C:\UTILS and the current working directory for F: is F:\BIN
You can verifiy this with the CD command:
F:\BIN> CD C:
C:\UTILS
F:\BIN>
Note that you did not change the working drive back to C: by using this command.
As you will see from the following command examples, the use of a backslash (\) immediately after a drive name (X:) makes the file location absolute. Ommiting the backslash automatically involves the current working directory for the drive.
F:\BIN> COPY F:PROGRAM.EXE C: Copies* F:\BIN\PROGRAM.EXE to C:\UTILS\PROGRAM.EXE
F:\BIN> COPY F:PROGRAM.EXE C:\ Copies* F:\BIN\PROGRAM.EXE to C:\PROGRAM.EXE
F:\BIN> COPY F:\PROGRAM.EXE C: Copies* F:\PROGRAM.EXE to C:\UTILS\PROGRAM.EXE
F:\BIN> COPY F:PROGRAM.EXE C:NEW\NEW_PROG.EXE Copies* and renames
F:\BIN\PROGRAM.EXE
to C:\BIN\NEW\NEW_PROG.EXE
* 'Copies' means 'attempts to copy'. These commands will fail if the assumed directory structures and current working directories don't exist or are changed by another process.
In the case where the working directory is the root directory (eg. C:\) then C:PROGRAM.EXE and C:\PROGRAM.EXE point to the same location, but they have been arrived at by different methods.
Best Answer
I am not a lawyer and this is not legal advice. This is simply my understanding of how this works within United States jurisdiction.
When you purchase "Microsoft Windows" or most other software, what you are actually purchasing is a license to use that software under the terms of its EULA. The software license you purchase permits you to use the software on one machine at a time. This is very similar to what you're buying when you purchase music or books; all of these are copyrighted works that the distributor is granting you a license to use with certain restrictions. Books and music usually come with the message "All rights reserved" and software usually comes with a EULA; essentially these permit you to use the work but not distribute additional copies of it.
In the interest of forcing consumers to follow this licensing policy, many software vendors make use of "product keys". Restrictions are built into the software that prevent its functioning unless you enter the product key. The vendor provides you with a product key when you purchase your license to use the software. Product keys are not a legal concept; they are a method of enforcing the terms of the license you purchased.
Some retail software doesn't make use of product keys, but this simply means the publisher hasn't chosen to enforce their EULA in this manner. You are still bound by the EULA even if the software isn't checking up on you.
So, to answer your question: Getting a licensed copy of Windows is the legal concept of purchasing the right to use the software, whereas the product key is the 25-character code meant to prove that you've done so.