OK, replacing OS files with older files is rarely ever a good idea and should be a last resort (and always take a restore point first).
From an command prompt run:
SFC /SCANNOW
It will likely have you reboot.
If "raspppoe.sys" is considered a system protected file then System File Checker (SFC) should restore it, but I'm never really sure which files are included on that list so if SFC doesn't fix it then hopefully you can manually restore the W7 version.
Now on to the most likely causes...
A virus is always a possibility. I really doubt it but running a complete scan with your A/V software is a good idea just to rule it out.
Most likely cause:
Do you have Windows Updates set for automatic updating? And do you use the default setting of "treat recommended updates the same as critical updates"?
Driver updates can be included as "recommended" updates and if you use the defaults then they will be included when windows updates are automatically installed. Unfortunately driver updates aren't always perfect and you can get a poorly written driver or quite simply a driver that was not meant for your device. In your situation this would most likely be the LAN driver for your wired port (wireless drivers are separate and you said your connection is wired).
Go to
Control Panel >
Device Manager. Expand the
Network Adapters. And double-click on the one for your wired connection. In the Properties window select the Driver Tab. Is the
Rollback Driver button available? If so click it. Now reboot and see if it is working.
If that works then you will want to prevent Windows Update from running again and putting the wrong driver back. To do that, run
Windows Updates manually. Click
Check for Updates. Then select the
optional updates. Find the driver and right-click. Choose
Hide Update.
If you never want drivers to be installed automatically you can disable that in your update settings. But if you do then you should manually run Windows Updates every couple months and decide whether there are any Recommended updates you do want.