Slow internet over LAN

I have recently connected a PC to a laptop, with the internet connection
being shared. The connection is directly to the PC, with the laptop
connected by an ethernet cable. The internet runs fine through the laptop,
but on the PC is extremely slow. I think this may be something to do with
the allocation of either memory or the internet connection. The PC has a
very slow internet connection even when the other computer is not switched
on, and the internet was fast until the LAN was set up. Any ideas?

Thanks
Ian