First, to answer "donc29's" questions, the "processes" listed in "Task Manager" are programs that load at start up and they are placed there during the installation of those programs and Wican be relatndows. "Processes" are not "services" but are certainly inter-connected. For example, the "svchost.exe" process mentioned in Donc29's question, loads at start up, and checks the services part of the registry to construct a list of services that it must load. Your antivirus needs to have a program listed in the "processes" tab, as do all other programs that are required to be running in the background during start up. Not all of the ones listed there are required. The program names you have listed are of that type:
osa.exe= Microsoft Office start up program, which for most users is unnecessary at start up and can easily be disabled and it will help avoid resource or conflict issues.
svchost.exe= The Service Host Process in Windows XP or Win2000, commonly seen in WinXP multiple times. Application that works as a host process for services that run from dynamic link libraries. Normally this is a valid Windows network connection host program, but it can be related to a number of viruses/trojans which will place the "svchost.exe" file in something other than the correct "C\Windows\System32" folder on WinXP. Naturually, if it's of the virus variety, you want to get rid of it.
ctsvccda= Background task for CD-ROM access that gets installed by the Windows 95, 98, or ME drivers of some Creative SoundBlaster soundcards. It also sometimes gets installed on Windows 2000 by non-driver related Creative software suites, such as Creative Jukebox.
alg.exe= Part of Internet Connection Sharing application and Internet Connection Firewall for Windows XP. This service provides support for third party protocol plug-ins for the Internet Connection Sharing application and Internet Connection Firewall. If you're not using the ICF or ICS in XP, this won't show up and can be disabled. It's not running on my home Win XP !
Depending on the computer, especially name brands, there may be a number of "processes" listed in the "Task Manager" that can be disabled at start up. It is correct to say that "many" of the processes are Windows System programs, but indicating that "Mcshield" is the one that slows down your computer may be totally incorrect. In fact, (and maybe I missed it here) did Donc29 tell us that McAfee Virus Scan was installed on his/her computer?
The sites mentioned by Cursorcowboy and RobertS will help donc29 identify those that are necessary, and those that are not. In addition to the other good responses, the link below should help you shut down some of those unnecessary start up programs:
How To Shut Down Unnecessary Start Up Programs
Hope this helps.