Can I backup a pc to a time capsule?

Potentially, you could use a Time Capsule for PC backups ... but it would require a Windows-based backup solution that supports backups to a network drive. You cannot use Time Machine for the PC. Note that there are similar apps for the PC.

One option I would suggest is to get an external USB drive that you can attach to the Time Capsule. Dedicate this drive for the PC backups.

Sometimes there are things you can do but the end result is probably you should NOT.

Is your PC laptop win10? The internal files backup is much improved over previous attempts MS has made at backup.

But the TC cannot support NTFS drives.. even if you connect a USB drive as Tesserax suggested I strongly recommend you keep the drive HFS+ (Mac OS extended journaled format). The airports can only recognise FAT32 and HFS+ format drives.. but FAT32 is very poor.. and over the network it makes no difference as either of them are FAT32 as the TC looks like NT server to the windows machine. Think back to when NT servers stopped and you have some idea of the issues.

The problems abound though.. due to FAT32 limitations with file size.. nothing above 4GB and using an Apple device which is really only discovered on the network via bonjour you should install bonjour for windows.. but Apple ceased supporting it at Win7. You can install it on later windows versions although it can behave poorly has no support.

Even if you use the TC for backup of your laptop incremental files.. let me recommend you make a disk image to a cheap portable USB drive so you have a way of recovering your windows without the complexity of network setups over DOS environments etc.