How Can I use my 21.5-inch late 2009 iMac as a monitor for my windows laptop?

You can't. The 21.5" Late 2009 iMac never supported Target Display Mode. Its Mini DisplayPort port is only for connecting an external monitor.

You can't. It' s an all in one computer, not a monitor. There is no video input. that model never supported Target Display Mode even when used with another Mac.

+1 Nope. If your Windows machine needs an external display, you simply need to buy one. Please remember iMacs are computers, they are NOT external displays for other devices. If your iMac had been the 27" version it may have worked assuming it was a late 2009 version.

Please refer to Apple's article on the subject called, Target Display Mode which has been dead for over a decade.

When COVID hit in 2020 and triggered a massive home-office need, internet pundits, lacking research skipps or editorial support, touted Target Display Mode (TDM) and doing everything from supporting Windows computers to parting the Red Sea. It was all post-processed horse feed.

The facts:

— Apple never intended, supported, nor documented using TDM-suported iMacs for any non-Apple devices.

— End users, not Apple, found almost by accident the some 27-inch (only) 2009 and 2012 iMac would do TDM from some PCs. That never applis etoany 21.5-inch iMac and no iMAcs aft ereh 2010.

The misinformation was so pervasive that we even heard reports here of Apple support and store employees telling customers TDM was still a "going thing" during COVID.

Apple started the deprecation of TDM in 2014, and threw the last shovel of dirt in its face early in 2020.

When Apple said in the Technical Specifications of the 27" Late 2009 and 27" Mid 2010 iMacs that these Macs accepted DisplayPort input, there were no qualifications saying that the source had to be a Mac.

From the Technical Specifications for the 27" Late 2009 iMac:

Apologies for the typos. I was in the process of editing when our power went out. "skipps" should be "skills."