Norton Anti Virus 2004 will boot from the NAV CD and check
your PC for viruses.
If it removes them and you can get it running, install NAV
then do your Definition Updates and re-scan.
You may have to set your BIOS to boot from CD.

A virus is always a possibility, but more info would be needed for others to
respond properly. How far does it boot before shutting down? Do any error
messages come up? be as specific as possible. It may very well be that something
internal is going south or there are loose/dirty connections. It could be the
motherboard, power supply, RAM, processor, fans, cables, etc. etc..

Yea, better update defs, & run a virus scan. Create a floppy-based scan
from another computer. After that...

(A)
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q188867
Troubleshooting Startup
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q187602
Troubleshoot
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q192926
Clean boot troubleshoot

(B)
http://support.microsoft.com/default.aspx?scid=kb;[LN];249191
Blank Desktop or Illegal Operations Error Message After You Install
Internet Explorer (This one temporarily reverts to an earlier version of
IE, IF you have that option. I have a detailed write-up, if you want
it.)

http://support.microsoft.com/?kbid=293174
Error Message: Explorer Caused an Invalid Page Fault in Module
Browseui.dll (This one is an over-install of your current IE.)

(1) Hold CTRL as you boot for the Startup Menu. Select to
boot to the "Command Prompt Only" (DOS).

(2) At DOS...

(a) Enter "EDIT  C:\Windows\System.ini"
(b) In the [Boot] section, change "Shell=Explorer.exe" to
"Shell=Progman.exe"
(c) Alt-F-X-ENTER (pressed separately) to exit Edit. (TAB to get to
buttons. ENTER to accept a button.)
(d) Ctrl-Alt-Del to Windows running Progman.

(3) In Progman, over-install IE, which basically is E. (I only hope it
doesn't ask for an internet connection. Perhaps, select "Custom" & only
check the items that have a bold checkbox, to avoid it will want the
internet. OTOH, it might be better to let it connect, if your DUN
allows.)

If you are IE6, "ie6setup.exe" will be in "C:\Windows\Windows Update
Setup Files". (You may also use the copy in "C:\Program Files\Internet
Explorer", which is "C:\Progra~1\Intern~1" to DOS.)

(a) In Progman, "File menu, Run, Browse button".
     Folders pane will be open at "C:\Windows" with sub-folders below
it. Otherwise, make it so.
(b) Scroll down the Folders pane & D-Clk "window~1".
     Now the left (File Name) pane should show "ie6setup.exe"
(c) D-CLK "IE6Setup.exe" to run it.
(d) Skip to step (4).

If you are IE5, "ie5setup.exe" will be in "C:\Windows Update Setup
Files". (You may also use the copy in "C:\Program Files\Internet
Explorer", which is "C:\Progra~1\Intern~1" to DOS.) I do not suppose you
should run "ie5setup.exe", if you have installed IE6. This one would be
only for those who never did install IE6.

(a) File menu, Run, Browse button.
     Folders pane will be open at "C:\Windows" with sub-folders below
it.
(b) D-Clk "C:\" in the Folders pane to go up one level.
(c) Scroll down the Folders pane & D-Clk "window~1".
(d) "ie5setup.exe" now appears in the left (File Name) pane.
(e) D-CLK "IE5Setup.exe" to run it.

(4) Reboot back to DOS. Then...

(a) Enter "EDIT  C:\Windows\System.ini"
(b) In the [Boot] section, change back to "Shell=Explorer.exe"
(c) Alt-F-X-ENTER to EXIT. (TAB to get to buttons. ENTER to accept a
button.)
(d) Ctrl-Alt-Del to Windows.