Error: Unsupported CPU installed for a HP Pavilion Slimline s3750t Desktop PC

[clarey526]

A/P this website https://support.hp.com/us-en/document/c01661175#AbT2 my older PC can be upgraded from Pentium E5300 (W) (2.6GHz /800 MHz) 2 MB L2 cache; Dual Core (65W)  to  Core 2 Quad Q9400 (Y) (2.66GHz /1333 MHz) 6 MB L2 cache; Quad Core (65W)... Right? Cause I tried doing this and I am now getting an Error: Unsupported CPU installed message at startup... Am I doing not understanding something? What am I doing wrong... I read through some other related posts on this matter... and all offer the advice to update bios but the last update for this PC was over 10yrs and even then the update was for Windows Vista and 7... I am currently using Windows 10 Home.

Should I just try to grade to Core 2 Duo E7500 (W) (2.93GHz /1066 MHz) 3 MB L2 cache; Dual Core (65W) and forget about Core 2 Quad upgrade... Please help?

Oh also, A/P the website... it claims that this PC can only use 4 GB (2 x 2 GB) DDR2 PC2-6400... Is that true?... Thanks for reading!

[Huffer]

HP seems to have pulled the Upgrade Manual for this model series. It's not impossible that the motherboards for Pentium and Dual Core might be different than the one used for Quad Core models. The spec page you link does not actually say you can upgrade it just says those were the factory options. Same with the memory. I would bet you could install a pair of 4 gig DDR2-800 aka PC2-6400 modules and they would work. However it is kind of an expensive experiment. Without definitive manuals and just working off my memory of that "era" that's the best I can do.