Thank you/ what can I do about this

First Id like to say I am truly thankful for all the help with all of the problems I have had in the past with my computers. When I start up explorer or Mozilla/ Firefox I get a message every time from my firewall (Sunbelt personal firewall) saying intrusion attempt blocked the list this. I run adware/ ccleaner/ avast antivirus none of them find anything.

Technical details about the intrusion attempt:
Injector application: <unknown>
Description: <unknown>
File version:
Product name:
Product version:
Created: N/A
Modified: N/A
Accessed: N/A
Target application: C:\WINDOWS\system32\lsass.exe
Description: LSA Shell (Export Version)
File version: 5.1.2600.2180 (xpsp_sp2_rtm.040803-2158)
Product name: Microsoft® Windows® Operating System
Product version: 5.1.2600.2180
Created: 2003/3/31, 12:00:00
Modified: 2004/8/4, 07:56:50
Accessed: 2007/9/28, 15:20:17
Address of injection: 0x7C801D77

Dell Inspiron 1300 notebook
512 mbs of ram
1.6 ghz
windows xp