Very slow internet and error message

Using Virgin (non -cable) through telephone, via BT line. Connection has
been very slow just lately with numerous sites 'not able to find'. Looked in
error log - I.P. address 192.168.0.2 not found by network address
OOOD56595ACB. Denied by DHCP 192.168.0.1 DHCPNACK message. Could this have
any relevance on our problem of slow connectivity and poor performance. Any
help and advice appreciated.
Fuzzyhead