Assuming you're behind a NAT like just about everyone is these days, probably very little.

Only TLS 1.0 works unless you install extras, so you'd be unable to load some sites. Microsoft in particular have turned it off, so you'll have trouble getting anything from them. The likes of Google and Facebook have it enabled still. It might be enabled for the update channels, but I don't have a box on hand to test.

Going off road might earn you a browser vuln from a dodgy video hosting site, since most browsers stopped getting updates a while ago.

Honestly your biggest problem is that machines still on XP are as old as the hills and about as quick. A modern website will bring a Pentium 4 to it's knees.