There are many alternatives to Fiddler for Windows if you are looking for a replacement. The best Windows alternative is Wireshark, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Fiddler and many of them are available for Windows so hopefully you can find a suitable replacement. Other interesting Windows alternatives to Fiddler are Charles (Paid), mitmproxy (Free, Open Source), OWASP Zed Attack Proxy (ZAP) (Free, Open Source) and Burp Suite (Freemium).
Wireshark is the world's foremost network protocol analyzer. It lets you capture and interactively browse the traffic running on a computer network. It is the de facto (and often de jure) standard across many industries and educational institutions.
Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).
Charles vs Fiddler opinions
Almost everyone thinks Charles is a great alternative to Fiddler.
NetworkMiner is a Network Forensic Analysis Tool (NFAT) for Windows. NetworkMiner can parse PCAP files and regenerate/reassemble transmitted files and certificates from PCAP files with HTTP, FTP, SMB, SMB2, TFTP and several other protocols.