This could be a virus, but in the meantime, try this: Type Ctl-Alt-Delete, and bring up the Task Manager.
The only programs that absolutely HAVE to be running are Explorer and Systray. Highlight each program that is running, one at a time, and click "End Task".
When you are finished, and left with only Explorer and Systray ... close the Task Manager, and attempt to restart normally.
If you are successful ... then the problem is likely to be one of those programs you manually turned off in the Task Manager.
If that is the case, go through the sequence again until you locate which program is causing the problem. Then either uninstall it, or change the options in the program so that it is not running continuously.
I suggest this ... because if the computer has been running for only thirty hours or so, it would be surprising for you to have already caught a virus (unless you have a penchant for opening email attachments!) You said yourself that the problem didn't occur until you started installing programs, tweaks, etc.
This should help you locate the "culprit".
Note: It's not likely to be a patch from the Windows Update Site ... I've upgraded Win98 many times from the site, and the only difficulty I've encountered is getting the dang java applet to download and install itself.
Note 2: This is NOT the only possible "fix". There could be a hardware conflict or a incorrect setting in the BIOS, and/or your system could be infected by a virus, as previously mentioned. Please add to this thread when you have the opportunity, and let me know if this solved your problem.
See ya ... Toejam31