Question : Ping time fluctuated

I have a WinXP connecting to 3com 2226 Plus - 3C16475BS switch, everyday my pc will have slowness problem, so did a ping, I ping to my router, the ping time is fluctuate, few lines <1ms, few lines = 84ms, few lines -1.
The weird thing is, after I reboot my pc, I will get back to normal ping time (< 1ms). But somehow after awhile, the issue comes again.
Any one can advise?

Reply from 192.168.0.1: bytes=32 time<1ms TTL=255
Reply from 192.168.0.1: bytes=32 time=87ms TTL=255
Reply from 192.168.0.1: bytes=32 time=87ms TTL=255
Reply from 192.168.0.1: bytes=32 time=87ms TTL=255
Reply from 192.168.0.1: bytes=32 time<1ms TTL=255
Reply from 192.168.0.1: bytes=32 time<1ms TTL=255
Reply from 192.168.0.1: bytes=32 time<1ms TTL=255
Reply from 192.168.0.1: bytes=32 time<1ms TTL=255

Answer : Ping time fluctuated

so it is PC problem (AV, Firewall, drivers, other soft...)