"winlogon.exe - entrypoint not found"

i have a laptop with win xp home installed - i recently
downloaded xp service pack 1 from ms website cos of all
viruses around now and seemed to install ok.
As there were 37 other xp security / upgrades that the ms
website said were available i chose some of them to
download (cant remember which) but while downloading then
got message that download / install had failed so turned
off computer normally without problem.

when tried to turn on again was unable to proceed further
than above error message. Message went on to say"the
procedure entry point AssocisDangerous could not be
located on the dynamic link library SHLWAPI.dll"

After clicking OK received second error message"user
interface failure- the logon user interface DLL msgina.dll
failed to load- contact your system administrator to
replace the dll or restore the original dll"

Help!!!!