First of all, download WinPatrol. Install it and then run it. Now, click on the Startup Programs tab, and look for the following programs -- if found, click on them once, and then choose Disable. This list are the programs that I either don't know / think that they could be the cause. It's up to you whether you wish to disable them or not -- this does not remove them, it simply makes them not startup...
8e6Home Internet Protection Services v.4.5
Access Manager
Audacity 1.2.3
Career Direct
Classic PhoneTools
Click to Meet Conference Client
DAO 3.5
Dell Modem-On-Hold
Dial Analysis
Intel® PRO Network Connections Drivers
ItsDeductible Express
PowerDVD
Quicken Basic 2000
QuickVerse 6.0
Rocket 2005 DEMO VERSION (1024x768) Screen Saver
Rocket DEMO VERSION (1024x768) Screen Saver
Shockwave
SoundMAX
SpaceCAD 3.1 (Build 284)
Symantec pcAnywhere
TurboTax Basic 2004
VCP6
VNC Free Edition 4.1.1
VPN Client
WeatherBug Browser Bar - powered by MyWebSearch
WexTech AnswerWorks
WordWeb
Once done, restart your computer and then try to share files -- can you?