No Applix? If not you might want to block the tcp/udp 999 service with tcp filter/firewall as I am hard pressed to find any useful service other than Applix.
Have you tried the online virus scan yet?
FYI, other known SubSevens that use port 999 - f0replay & WinSatan.
Check the Run/RunServices keys in the reg under HKCU hive. Also check your CONFIG.SYS, AUTOEXEC.BAT, WINSTART.BAT, WIN.INI, for any strange apps loading.
Let us know........