I "upgraded" to IE8 on our PC (older one with a Pentium 4 and XP Media Center) which previously worked great (if getting a little slow). After the "upgrade" it wouldn't even boot, giving a "missing or corrupt hal.dll file" error. I bought a "rescue" software package and got the thing to at least boot, but no manner of uninstalling IE8, reinstalling IE7, or SP2 or SP3 or anything would get IE up and running in any version. At least the computer was otherwise functional and I use Firefox anyway, but still it pi$$ed me off. I spent a weekend on it before I finally just restored the entire system from about a 6 week old backup after saving off any new files since that date to the USB hard drive.
After the restore, all was well again, but after I looked around, it seems there are a LOT of issues with IE8, so I'm staying real far away from that POS!