In general, Macs and PCs play fairly nicely together. Most file types work on both platforms, and both will talk to each other across a network.
However, if you have an portable hard drive for your Mac, it can be tricky to share it with a Windows device as it's likely to be using Apple's HFS+ or the latest APFS file system, neither of which Windows supports natively.
Apple introduced NTFS support in OS X Leopard so you can share a Windows drive with your Mac easily enough. And there's a remote possibility your portable drive uses exFAT or FAT32 which both operating systems can read. But it probably doesn't.
Now, the good news is that you can get a variety of apps including Paragon APFS (£43.99 / $49.95) or MacDrive (£35 / $49.99) which will read and write drives formatted with APFS. MacDrive also supports HFS+. Paragon also has HFS+ for Windows which can read and write to HFS-formatted drives. It costs around £15 / $19.95.
The bad news is that there are no free apps which will let you connect an APFS-formatted drive to Windows and read the contents for free.
But if your drive is formatted with the older HFS, then you can read it in Windows without spending any money.
How to read files from an HFS+ hard drive in Windows
First, download HFSExplorer. It supports HFS, HFS+ and HFSX file systems.
Make sure you download the Windows Installer, as this comes with the necessary Java SE Runtime Environment. Install the program using the default settings. You'll end up with a new program in your Start menu: HFSExplorer.
Connect your hard drive and, if appropriate, the power supply. Run HFSExplorer and ignore any message that a DLL file is missing.
The next step is to select the hard disk by clicking File > Load file system from device. If you have only one hard disk inside your PC or laptop, it's likely that your USB drive will appear as Harddisk1/Partition1, but it may be Harddisk2/Partition1. If your USB disk has more than one partition, you'll need to choose the one where the files you want are stored.
When you've selected the right disk, you should see its contents displayed in a Windows Explorer-like tree structure.
In order to copy files to your laptop or PC's hard disk, select the ones you want and click the Extract button. Browse to a folder on your local hard disk (or a different external drive) and click Extract here. Note that you can't create a new folder at this point, so you'll have to choose an existing one.
The copy process will begin and, when complete, you'll be able to open the files in Windows.