There are several things to look at, first, try setting the speed/duplex on all devices, including the switch to to 100/full. If you can, look at the interface on the switch and see if there are errors or packet drops. Remove any unnecessary protocols, only use TCP/IP. When you ping the other PC it should be less than 10ms for the round trip time. Lastly, if all else fails, download and install Wireshark from www.wireshark.org. This is a free and excellent network sniffer. You can capture the packets with this and see if there are retransmits or errors on the wire.