If you've checked, as suggested above, to make sure you haven't just changed the individual Window to a max from your usual part-screen display, you should take a look at whether the driver for the monitor has been changed. It's remotely possible that one of your AV programs thought it found one of the extremely rare "driver worms" and when it removed the worm it corrupted the driver. If Windows found a bad or missing driver, it may have substituted a generic one.
I don't have a Win98 system up, but in Control Panel, right click on Display and select properties and it may tell you what's being used. You may need to double-click on Display to open it up fully, and look for something that identifies what graphic card and/or monitor it's set up for. Of course you will need to have some idea of what's supposed to be there, and if that's a mystery you can probably safely skip this step.
It's unlikely that you'll find anything out of order there, but if you do find someting that doesn't look right you'll probably need to make changes in Device Manager/Hardware Manage. If you need help with that, ask.
While you're in Control Panel | Display, you should check to see what screen resolution you're set up for. Some programs, often games - especially those you play "on the web" - may change your screen resolution and may fail to put it back when you exit the program. Such programs occasionally set up a display resolution that's not appropriate for your graphics card and monitor. If you look at it in Control Panel | Display, it should offer only choices appropriate for your monitor/graphics card.
In the same place, Control Panel | Display, you can check to see if appearance settings may have changed. You should have choices for font sizes, colors, icon size (and spacing) etc. If all else fails, there should be "default" buttons on most settings that will set them back to some "original Windows" selections.
Changes in software aren't supposed to change monitor setup, but occasionally they will. If the full Windows display doesn't fit in the visible part of the monitor (the display - not just an individual window - runs off the side of the physical screen or pulls in excessively from the side) you may need to make "monitor adjustments." When you reboot, PlugNPlay should select the right driver for your monitor, and it should come pretty close to displaying the right size, but sometimes adjustments are needed even with the right drivers, and the settings needed may change with age of the monitor.
Some Windows OS setups will put a "monitor" button in the System Tray at the lower right corner of the Windows screen. Hover your mouse over any that are there, and you should get a popup flag to tell you what each one is. If you find one there that looks like it might be a "display" control, double click it to open it up and see if it offers you any screen adjustments.
If there isn't a display manager there, your monitor probably has a bunch of mysterious buttons somewhere, usually at the bottom below the display, and sometimes behind a little trap door. There is quite a bit of variation in what buttons are there, and no known standard for how they'll be labeled, but these buttons can be used to change the physical size and centering of the "picture" on the monitor face. There usually are adjustments for size, vertical position, horizontal position, pincushion/barreling, and skew. Often there's an adjustment for "color temperature" and sometimes brightness/contrast. These are "mechanical" adjustments of the stuff inside the monitor.
If you decide to make adjustments there, you should close as much stuff as possible on the display so that you see only the full clean display of the Windows desktop. Usually a "status" or "setup" button on the monitor will open a display with a bunch of choices. You move to the choice you want, sometimes by using buttons on the monitor, sometimes by using the Tab button on the keyboard and sometimes by using the arrow keys on the keyboard. Usually there are buttons on the monitor for Adjust +/- after you get to the function you want to adjust. Sometimes there will be a "Save and Exit" in the on-screen display, or there may be a "Save" button among those on the monitor. Usually you have to deliberately "Save" any changes you make, and if you exit without saving your changes won't be made.
I've never heard of any serious problems with going into the buttons and using the "flounder until found" method, but if it makes you nervous you may be able to find setup instructions at the monitor maker's site on the web.
The display - the part of the monitor face that's lit up - should be squared up, with straight sides. There should be a small dark border visible all the way around it. Avoid excessive brightness/contrast settings, but get something that's "comfortable" for your viewing situation if there's an adjustment for it.
The most likely place to fix what's bothering you is in the Control Panel | Display | Appearance settings setup.
A remote possibility is that you've turned on one of the "Accessibility Options," that sets "extra large type" for visually challenged users, although your description doesn't really sound like that's the case.