Depends how you want to do it. Synergy is like a software KVM for input only, so you control the PC on its own display, not on your Mac's display. Once you set it up it's pretty easy, and works like an extended desktop a bit (you can drag your mouse over to the PC screen but not windows and apps). But if you want to control the PC on your mac display (in a window, or full screen) then you can use remote desktop or vnc.
Remote desktop server is built into windows (you may need to enable it, and it is not available on some editions of Windows, i.e. the non-"pro" ones) and they have a free client for the mac you can download. If this is available to you and this is what you want to do (control the PC on your mac display) then this is what I would do for a PC.
Alternately VNC server is built into all modern macs, and you'd want a VNC client to connect to it. To connect from your mac to PC with VNC you need a VNC client on your mac and a VNC server on the PC. If your Windows version does not support remote desktop then this is what I would use.
All the above is free, but VNC tries to make free hard to find, so lemme know if you need a link to the free versions. The setup to all three can also be tricky, especially cross platform so let me know if you run into any walls.
P.S. you can then get rd/vnc clients for iOS (but not free) and control your PC or mac from your iPod touch etc.