Ability to open short threads but not long ones suggests a temp space problem.
If you've rebooted recently, most temp files should have been cleared, although odd settings can let some remain. (People with more than one browser need to make sure that all of the browsers are set to delete Temp Internet Files on closing.)
The place to check is to right click on your system hard drive in Windows Explorer, click Properties, and then click on Disk Cleanup (or something similar depending on how old your Microsaurus is). You get the same menu as in IE at Tools|Internet Options, but open files can't be deleted so doing it at the drive, with IE closed, may pick up a few more files than doing it in IE. Ideally you'll want to close as many windows as possible before doing the disk cleanup.
Disk cleanup should get most of the trash, but since it "inspects" to make a list of what can be removed, and lets you uncheck the ones you don't want removed, you may have accidentally unchecked something that you don't really need. Disk Cleanup will keep the checkmarks for whatever items were checked the last time it ran, so once something is changed it stays changed until you unchange it. Look at the list and make sure that at least Temporary Internet Files and Temporary Files (different on some Windows versions) and Recycle Bin are checked before you OK to do the actual cleanup.
While you're at the drive "Properties" take a look at how big your hard drive is and how much free space it shows. Most older versions limited the "Temp" space to around 10% of the free space on the drive, so even if you've got e.g. a Gigabyte free you'll only be able to use 100 Megabytes. If you're down to a few Megabytes free, your maximum temp space may be only a few KB.
If Microsaurus is really old, you may run into the problem that some older versions can only use the largest single contiguous free space on the hard drive. The Temp file area can't jump from one free space area to the next on these older systems. Defrag mostly aims at rearranging the files, and lets the free space be "wherever," but sometimes repeating Defrag a couple of times will also "defrag the free space" so that you get a bigger space for the Temp file. Once the free space is "defragged" you may need to go back and clear all the temp files again before the Temp folder(s) can relocate themselves.
If hard drive space looks marginal, note that "previews" that Win Explorer makes to show mini-images of image files can take quite a bit of space. It's not necessary to keep them, since Explorer can re-create any that you look at as "icons;" but of course if it's necessary to delete them to solve the problem, the problem will come back the next time you look at those files (with icons on).
It wouldn't hurt to look at your recycle bin and empty it if needed, even if Disk cleanup said it cleaned it.
These cleanups may or may not help; but they're pretty simple to run through - with defrag being the only really time consuming (sometimes) part of it.