Q: Can I attach a PC laptop to my iMac MB417LL/A to use the Mac screen as a display

unfortunately you can't, because your iMac cannot be used as simple external monitor because does not have multiple exits like a simple External monitor.

Feb. 2016 https://discussions.apple.com/message/29761388#29761388 - "Whether or not you can use your older iMac as a secondary display for your new iMac (known as Target Display Mode) will depend on the specific model. "

No, Use your iMac as a display with Target Display Mode - Apple Support

Well Here is the good news:)

Actually you can but you must have a thunderbolt output in your laptop or at least (DisplayPort)

I'm right now running my imac 27 inch (2009) late in my pc with gtx 970 graphics and it looks very awesome!

I used MiniDisplayPort to DisplayPort cable, bought it from amazon for 9.99$

Cheers,

Msh3lany,

Your 27" iMac has that ability, The OP's 20" iMac does not have this ability.