You don't say whether the old PC, which works fine, is connecting to the router by ethernet cable or wirelessly.
If it's only the new PC which is "wireless", try changing the Channel number of the wireless network from the default setting of 11. Change it to something like 6 or 3 and see if that makes a difference.
Make sure there are no mobile or cordless phones close to either the router or the PC's wireless adapter. Don't locate the router or PC close to large metal objects, in particular central heating radiators.